[libc] Implement clock_getcpuclockid on Linux. (#218531)

Implement `clock_getcpuclockid` POSIX function by mimicking the Linux
kernel conversion of pid_t to the clockid_t, and validating the result /
permissions with `clock_getres` syscall. Add the corresponding syscall
wrapper for the latter.

GitOrigin-RevId: 3439b12763b2411773cd3267e6c3b67d584082e6