mcheck: Align struct hdr to MALLOC_ALIGNMENT bytes [BZ #28068]

1. Align struct hdr to MALLOC_ALIGNMENT bytes so that malloc hooks in
libmcheck align memory to MALLOC_ALIGNMENT bytes.
2. Remove tst-mallocalign1 from tests-exclude-mcheck for i386 and x32.
3. Add tst-pvalloc-fortify and tst-reallocarray to tests-exclude-mcheck
since they use malloc_usable_size (see BZ #22057).

This fixed BZ #28068.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>