Handle the three potential system scenarios:
- system time_t is time64
- system time_t is time32 and supports time64
- system time_t is time32 and does not support time64
Add the explicit time32 and time64 functions when necessary and map
them accordingly for each of these cases.
This is a non-portable header, and we cannot expect it to be provided by
the system libc (e.g. musl). We just need and rely on declaration that
we have defined ourselves in our own <bsd/sys/cdefs.h>. So we switch to
only ever assume that.
Fixes: https://bugs.freedesktop.org/105281
