posix: implement the POSIX_CLOCK_SELECTION Option Group

Implement the POSIX_CLOCK_SELECTION Option Group.

This was mostly already done, but compiled / linked in the wrong places.

E.g. pthread_condattr_getclock() and pthread_condattr_setclock() were
in pthread.c and part of POSIX_THREADS_BASE. clock_nanosleep() was in
clock.c and part of POSIX_TIMERS.

This change builds them as part of clock_selection.c with
CONFIG_POSIX_CLOCK_SELECTION.

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