sys: clock: additional sys_clock api calls

Additional entries for the sys_clock API, comprised of:

* sys_clock_gettime()
* sys_clock_settime()
* sys_clock_nanosleep()

along with the constants

* SYS_CLOCK_REALTIME
* SYS_CLOCK_MONOTONIC
* SYS_TIMER_ABSTIME

The primary motivation for this API is so that libc and other
libraries have a familiar-enough API to reach to when POSIX
is not available, since POSIX is optional in Zephyr.

By adding this API to lib/os, we also eliminate dependency cycles
between libc and posix, as lib/os is a mutual dependency.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>