* m4/stddef_h.m4 (gl_STDDEF_H): Test the alignment of max_align_t also
on clang.
* lib/alignof.h (alignof_type): Use __alignof__ also on clang.
* lib/stdalign.in.h (_Alignof): Don't activate the GCC workaround on
clang.
* lib/malloca.h (sa_alignof): Use __alignof__ also on clang.
* lib/bitset/list.c (lbitset_elt_alloc): Use __alignof__ also on clang.
* lib/bitset/table.c (tbitset_elt_alloc): Likewise.
* tests/test-stddef.c: Very the behaviour of __alignof__ also on clang.