Implement C23 memalignment

Add the C23 memalignment function (query the alignment of a pointer)
to glibc.

Given how simple this operation is, it would make sense for compilers
to inline calls to this function, but I'm treating that as a compiler
matter (compilers should add it as a built-in function) rather than
adding an inline version to glibc headers (although such an inline
version would be reasonable as well).  I've filed
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122117 for this feature
in GCC.

Tested for x86_64 and x86.

stdlib/stdlib.h

@@ -1164,6 +1164,9 @@ extern int ttyslot (void) __THROW;
 /* Call function __FUNC exactly once, even if invoked from several threads.
    All calls must be made with the same __FLAGS object.  */
 extern void call_once (once_flag *__flag, void (*__func)(void));
+
+/* Return the alignment of P.  */
+extern size_t memalignment (const void *__p);
 #endif
 
 #include <bits/stdlib-float.h>