Expose POSIX functions incorporated into C23

C23 has incoporated a small number of POSIX functions, so these
should be exposed for C23, regardless of POSIX.

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D47856

(cherry picked from commit 59677aecb67bbedcfa2ee5d7d2b189193cdc4af7)