cdefs: Support _ISOC23_SOURCE and --std=c23

If either _ISOC23_SOURCE is defined or --std=c23 (or higher) is
specified, then default to the making the ISO C visibility to 2023.
This mirrors what glibc does, so update the comment for this change to
_ISOCxx_SOURCE values. We currently implement xx = 11 or 23. C17 added
no new defines or symbols, so we follow glibc's lead and ommit it.
However, we don't implement the C95, C99 or C2y versions. These are
non-standard and the first two don't seem to be relevant, and the latter
is also experimental.

Sponsored by: Netflix
Reviewed by: brooks
Differential Revision: https://reviews.freebsd.org/D47576