include/stdbit.h: declare size_t, (u)int*_t, and (u)int_least*_t

These are required by ISO/IEC 9899:2024 § 7.18.1 ¶ 1 but were forgotten
in my initial work.

The current approach leaks intptr_t, uintptr_t, intmax_t, and uintmax_t
through <sys/_stdint.h>. This could be avoided using a more complicated
approach if desired.

PR: 294131
Fixes: 6296500a85c8474e3ff3fe2f8e4a9d56dd0acd64
Reported by: Collin Funk <collin.funk1@gmail.com>
Reviewed by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D56515