libstdc++: Prefer posix_memalign for aligned-new [PR113258]

As described in PR libstdc++/113258 there are old versions of tcmalloc
which replace malloc and related APIs, but do not repalce aligned_alloc
because it didn't exist at the time they were released. This means that
when operator new(size_t, align_val_t) uses aligned_alloc to obtain
memory, it comes from libc's aligned_alloc not from tcmalloc. But when
operator delete(void*, size_t, align_val_t) uses free to deallocate the
memory, that goes to tcmalloc's replacement version of free, which
doesn't know how to free it.

If we give preference to the older posix_memalign instead of
aligned_alloc then we're more likely to use a function that will be
compatible with the replacement version of free. Because posix_memalign
has been around for longer, it's more likely that old third-party malloc
replacements will also replace posix_memalign alongside malloc and free.

libstdc++-v3/ChangeLog:

	PR libstdc++/113258
	* libsupc++/new_opa.cc: Prefer to use posix_memalign if
	available.