x86-vxworks malloc aligns to 8 bytes like solaris

x86-vxworks malloc aligns to 8 bytes like solaris

Vxworks 7's malloc, like Solaris', only ensures 8-byte alignment of
returned pointers on 32-bit x86, though GCC's stddef.h defines
max_align_t with 16-byte alignment for __float128.  This patch enables
on x86-vxworks the same memory_resource workaround used for x86-solaris.

The testsuite also had a workaround, defining BAD_MAX_ALIGN_T and
xfailing the test; extend those to x86-vxworks as well, and remove the
check for char-aligned requested allocation to be aligned like
max_align_t.  With that change, the test passes on x86-vxworks; I'm
guessing that's the same reason for the test not to pass on
x86-solaris (and on x86_64-solaris -m32), so with the fix, I'm
tentatively removing the xfail.

for libstdc++-v3/ChangeLog

	PR libstdc++/77691
	* include/experimental/memory_resource
	(__resource_adaptor_imp::do_allocate): Handle max_align_t on
	x86-vxworks as on x86-solaris.
	(__resource_adaptor_imp::do_deallocate): Likewise.
	* testsuite/experimental/memory_resource/new_delete_resource.cc:
	Drop xfail.
	(BAD_MAX_ALIGN_T): Define on x86-vxworks as on x86-solaris.
	(test03): Drop max-align test for char-aligned alloc.