stdalign: relax _Alignof and tighten _Alignas test

* m4/stdalign.m4 (gl_STDALIGN_H): Relax the _Alignof test,
as it was too strict: alignof must divide offsetof, but it need
not equal offsetof.  Inspired by Joseph S. Myers's comment
<http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023#c10>.
Conversely, tighten the _Alignas test a bit, as the resulting
alignment must be exactly 8.