Reduce the memory overhead of sampled small allocations

Previously, small allocations which were sampled as part of heap
profiling were rounded up to `SC_LARGE_MINCLASS`. This additional memory
usage becomes problematic when the page size is increased, as noted in #2358.

Small allocations are now rounded up to the nearest multiple of `PAGE`
instead, reducing the memory overhead by a factor of 4 in the most
extreme cases.

src/pages.c

@@ -33,7 +33,7 @@
 /* Data. */
 
 /* Actual operating system page size, detected during bootstrap, <= PAGE. */
-static size_t	os_page;
+size_t	os_page;
 
 #ifndef _WIN32
 #  define PAGES_PROT_COMMIT (PROT_READ | PROT_WRITE)