Use max_align_t from <stddef.h>.

Use max_align_t from <stddef.h>.

Now that we build with -std=gnu11 and can rely on a compiler providing
max_align_t in <stddef.h>, we no longer need our own version
libc_max_align_t.  This patch removes it and replaces the single user
with a use of max_align_t.

Tested for x86_64 and x86 (testsuite, and that installed stripped
shared libraries are unchanged by the patch for x86_64; for x86, I see
some code reordering of no significance).

	* include/libc-internal.h (libc_max_align_t): Remove typedef.
	* include/scratch_buffer.h: Include <stddef.h> instead of
	<libc-internal.h>.
	(struct scratch_buffer): Use max_align_t instead of
	libc_max_align_t.