libgomp: Fix up aligned_alloc arguments [PR102838]

C says that aligned_alloc size must be an integral multiple of alignment.
While glibc doesn't care about it, apparently Solaris does.
So, this patch decreases the priority of aligned_alloc among the other
variants because it needs more work and can waste more memory and rounds
up the size to multiple of alignment.

2021-11-18  Jakub Jelinek  <jakub@redhat.com>

	PR libgomp/102838
	* alloc.c (gomp_aligned_alloc): Prefer _aligned_alloc over
	memalign over posix_memalign over aligned_alloc over fallback
	with malloc instead of aligned_alloc over _aligned_alloc over
	posix_memalign over memalign over fallback with malloc.  For
	aligned_alloc, round up size up to multiple of al.