posix: clock: clock_gettime() should not be a syscall

We try to implement Zephyr's POSIX API as regular library
functions, so remove the __syscall annotation from
clock_gettime() and implement the syscall portion of it under
the hood.

This also adds a bit of a micro-optimization in that
we can do a lot of processing outside of the system call.

In fact, processing CLOCK_MONOTONIC likely does not
require any syscall other than k_uptime_ticks().

Signed-off-by: Christopher Friedt <cfriedt@meta.com>