Support C++17 over-aligned allocation

Summary:
Add support for C++17 over-aligned allocation:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0035r4.html

Supporting all 10 operators means we avoid thunking thru libstdc++-v3/libsupc++ and just call jemalloc directly.

It's also worth noting that there is now an aligned *and sized* operator delete:
```
void operator delete(void* ptr, std::size_t size, std::align_val_t al) noexcept;
```

If JeMalloc did not provide this, the default implementation would ignore the size parameter entirely:
https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/libsupc%2B%2B/del_opsa.cc#L30-L33

(I must also update ax_cxx_compile_stdcxx.m4 to a newer version with C++17 support.)

Test Plan:
Wrote a simple test that allocates and then deletes an over-aligned type:
```
struct alignas(32) Foo {};
Foo *f;

int main()
{
  f = new Foo;
  delete f;
}
```

Before this change, both new and delete go thru PLT, and we end up calling regular old free:
```
(gdb) disassemble
Dump of assembler code for function main():
...
   0x00000000004029b7 <+55>:    call   0x4022d0 <_ZnwmSt11align_val_t@plt>
...
   0x00000000004029d5 <+85>:    call   0x4022e0 <_ZdlPvmSt11align_val_t@plt>
...
(gdb) s
free (ptr=0x7ffff6408020) at /home/engshare/third-party2/jemalloc/master/src/jemalloc.git-trunk/src/jemalloc.c:2842
2842            if (!free_fastpath(ptr, 0, false)) {
```

After this change, we directly call new/delete and ultimately call sdallocx:
```
(gdb) disassemble
Dump of assembler code for function main():
...
   0x0000000000402b77 <+55>:    call   0x496ca0 <operator new(unsigned long, std::align_val_t)>
...
   0x0000000000402b95 <+85>:    call   0x496e60 <operator delete(void*, unsigned long, std::align_val_t)>
...
(gdb) s
116             je_sdallocx_noflags(ptr, size);
```

src/jemalloc_cpp.cpp

@@ -39,6 +39,20 @@ void	operator delete(void *ptr, std::size_t size) noexcept;
 void	operator delete[](void *ptr, std::size_t size) noexcept;
 #endif
 
+#if __cpp_aligned_new >= 201606
+/* C++17's over-aligned operators. */
+void	*operator new(std::size_t size, std::align_val_t);
+void	*operator new(std::size_t size, std::align_val_t, const std::nothrow_t &) noexcept;
+void	*operator new[](std::size_t size, std::align_val_t);
+void	*operator new[](std::size_t size, std::align_val_t, const std::nothrow_t &) noexcept;
+void	operator delete(void* ptr, std::align_val_t) noexcept;
+void	operator delete(void* ptr, std::align_val_t, const std::nothrow_t &) noexcept;
+void	operator delete(void* ptr, std::size_t size, std::align_val_t al) noexcept;
+void	operator delete[](void* ptr, std::align_val_t) noexcept;
+void	operator delete[](void* ptr, std::align_val_t, const std::nothrow_t &) noexcept;
+void	operator delete[](void* ptr, std::size_t size, std::align_val_t al) noexcept;
+#endif
+
 JEMALLOC_NOINLINE
 static void *
 handleOOM(std::size_t size, bool nothrow) {
@@ -76,12 +90,46 @@ JEMALLOC_ALWAYS_INLINE
 void *
 newImpl(std::size_t size) noexcept(IsNoExcept) {
 	void *ptr = je_malloc(size);
-	if (likely(ptr != nullptr))
+	if (likely(ptr != nullptr)) {
 		return ptr;
+	}
 
 	return handleOOM(size, IsNoExcept);
 }
 
+template <bool IsNoExcept>
+JEMALLOC_ALWAYS_INLINE
+void *
+alignedNewImpl(std::size_t size, std::align_val_t alignment) noexcept(IsNoExcept) {
+	void *ptr = je_aligned_alloc(static_cast<std::size_t>(alignment), size);
+	if (likely(ptr != nullptr)) {
+		return ptr;
+	}
+
+	return handleOOM(size, IsNoExcept);
+}
+
+JEMALLOC_ALWAYS_INLINE
+void
+sizedDeleteImpl(void* ptr, std::size_t size) noexcept {
+	if (unlikely(ptr == nullptr)) {
+		return;
+	}
+	je_sdallocx_noflags(ptr, size);
+}
+
+JEMALLOC_ALWAYS_INLINE
+void
+alignedSizedDeleteImpl(void* ptr, std::size_t size, std::align_val_t alignment) noexcept {
+	if (config_debug) {
+		assert(((size_t)alignment & ((size_t)alignment - 1)) == 0);
+	}
+	if (unlikely(ptr == nullptr)) {
+		return;
+	}
+	je_sdallocx(ptr, size, MALLOCX_ALIGN(alignment));
+}
+
 void *
 operator new(std::size_t size) {
 	return newImpl<false>(size);
@@ -102,6 +150,30 @@ operator new[](std::size_t size, const std::nothrow_t &) noexcept {
 	return newImpl<true>(size);
 }
 
+#if __cpp_aligned_new >= 201606
+
+void *
+operator new(std::size_t size, std::align_val_t alignment) {
+	return alignedNewImpl<false>(size, alignment);
+}
+
+void *
+operator new(std::size_t size, std::align_val_t alignment, const std::nothrow_t &) noexcept {
+	return alignedNewImpl<true>(size, alignment);
+}
+
+void *
+operator new[](std::size_t size, std::align_val_t alignment) {
+	return alignedNewImpl<false>(size, alignment);
+}
+
+void *
+operator new[](std::size_t size, std::align_val_t alignment, const std::nothrow_t &) noexcept {
+	return alignedNewImpl<true>(size, alignment);
+}
+
+#endif  // __cpp_aligned_new
+
 void
 operator delete(void *ptr) noexcept {
 	je_free(ptr);
@@ -125,17 +197,46 @@ void operator delete[](void *ptr, const std::nothrow_t &) noexcept {
 
 void
 operator delete(void *ptr, std::size_t size) noexcept {
-	if (unlikely(ptr == nullptr)) {
-		return;
-	}
-	je_sdallocx_noflags(ptr, size);
+	sizedDeleteImpl(ptr, size);
 }
 
-void operator delete[](void *ptr, std::size_t size) noexcept {
-	if (unlikely(ptr == nullptr)) {
-		return;
-	}
-	je_sdallocx_noflags(ptr, size);
+void
+operator delete[](void *ptr, std::size_t size) noexcept {
+	sizedDeleteImpl(ptr, size);
 }
 
 #endif  // __cpp_sized_deallocation
+
+#if __cpp_aligned_new >= 201606
+
+void
+operator delete(void* ptr, std::align_val_t) noexcept {
+	je_free(ptr);
+}
+
+void
+operator delete(void* ptr, std::align_val_t, const std::nothrow_t&) noexcept {
+	je_free(ptr);
+}
+
+void
+operator delete[](void* ptr, std::align_val_t) noexcept {
+	je_free(ptr);
+}
+
+void
+operator delete[](void* ptr, std::align_val_t, const std::nothrow_t&) noexcept {
+	je_free(ptr);
+}
+
+void
+operator delete(void* ptr, std::size_t size, std::align_val_t alignment) noexcept {
+	alignedSizedDeleteImpl(ptr, size, alignment);
+}
+
+void
+operator delete[](void* ptr, std::size_t size, std::align_val_t alignment) noexcept {
+	alignedSizedDeleteImpl(ptr, size, alignment);
+}
+
+#endif  // __cpp_aligned_new