libthr mutex_self_lock(): handle signals and clock steps

[kib]

Summary

Relocking an owned NORMAL mutex with an absolute timeout has two issues:
- nanosleep(2) is never restarted, so any caught signal causes
  an early ETIMEDOUT.
- If CLOCK_REALTIME or CLOCK_TAI are stepped backward during the sleep,
  the call also returns ETIMEDOUT before the clock reaches the deadline.

The kernel already polls CPU-time clocks and handles realtime steps.
Use _thr_umtx_timedwait_uint() in a loop to handle both issues.