alignasof: new module

This splits off support for the C23 keywords alignas and alignof,
from the now-deprecated stdalign module.  The latter now merely
provides C11 support.
* MODULES.html.sh, NEWS, doc/gnulib.texi:
* doc/posix-headers/stdalign.texi: Document the change.
* lib/stdalign.in.h: Remove most of the definitions (which are now
supplied by the alignasof module), leaving only
__alignas_is_defined and __alignof_is_defined.
* modules/alignasof, modules/alignasof-tests: New files.
* m4/stdalign.m4 (gl_ALIGNASOF): New macro, with
most of the contents of the old gl_STDALIGN_H.
Do not define __alignas_is_defined or __alignof_is_defined.
(gl_STDALIGN_H): Rely on gl_ALIGNASOF for most of the work.
* modules/alignalloc, modules/alignof, modules/argp:
* modules/crypto/md4-buffer, modules/crypto/md5-buffer:
* modules/crypto/sha1-buffer, modules/crypto/sha256-buffer:
* modules/crypto/sha512-buffer, modules/crypto/sm3-buffer:
* modules/fts, modules/rawmemchr, modules/relocatable-prog-wrapper:
* modules/stddef-tests, modules/sys_socket:
Depend on alignasof, not stdalign.
* modules/stdalign: Deprecate.  Depend on alignasof.
* modules/stdalign-tests: Move most contents to the new module
alignasof-tests, and depend on that.

modules/alignalloc

@@ -7,12 +7,12 @@ lib/alignalloc.c
 m4/alignalloc.m4
 
 Depends-on:
+alignasof
 assert-h
 extensions
 extern-inline
 idx
 posix_memalign
-stdalign
 stdckdint
 stdint