libc: implement C23 <stdbit.h> functions

This new header complies with ISO/IEC 9899:2024 (C23).

Contrary to glibc, we do not provide inline definitions in
<stdbit.h> as we expect our system compiler to soon recognise
these as builtins anyway.

Relnotes: yes
MFC after: 1 month
Reviewed by: adrian
Approved by: markj (mentor)
Differential Revision: https://reviews.freebsd.org/D53657