libc: common: time: use sys_clock api rather than posix

Remove POSIX clock_gettime() from the common libc time implementation,
since POSIX should not be a dependency for ISO C.

Instead, use the newly added lib/os sys_clock API.

Specifically, sys_clock_gettime().

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