Replace std::__throw_bad_alloc call with standard C++ (#2900)

* Replace std::__throw_bad_alloc call with standard C++

Since December of 2025, std::__throw_bad_alloc is no longer visible
through #include <new> causing jemalloc build failures with gcc 16.
As far as I can tell, all std::__throw_bad_alloc did was arrange to
raise a std::bad_alloc exception if exceptions are enabled.  I am not
sure whether its usage was truly meaningful in jemalloc since the call
is wrapped in a try catch and any usage of try catch is considered an
error when compiling with -fno-exceptions on gcc, at least.

This change adds a check to configure.ac that determines whether
exceptions are enabled by compiling a simple try catch that raises a
std::bad_alloc exception.  If that test succeeds, the macro
JEMALLOC_HAVE_CXX_EXCEPTIONS is defined, and jemalloc will raise an
exception.  Otherwise, we call std::terminate() to abort.

This was tested on FreeBSD with the gcc16 port with and without exceptions
enabled.

* Replace std::set_new_handler calls with std::get_new_handler

Previously, std::set_new_handler was used as a workaround for
compilers with only partial support for C++11.  Now that C++14 is a
requirement to enable C++ support, we can assume std::get_new_handler
is available.

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -465,6 +465,9 @@
 /* Is C++ support being built? */
 #undef JEMALLOC_ENABLE_CXX
 
+/* Are C++ exceptions enabled? */
+#undef JEMALLOC_HAVE_CXX_EXCEPTIONS
+
 /* Performs additional size checks when defined. */
 #undef JEMALLOC_OPT_SIZE_CHECKS