posix: implement pthread_condattr_setclock()

[ycsin]

There are currently 20 missing POSIX functions with trivial implementations, pthread_condattr_setclock() is one of them.

pthread_condattr_getclock, pthread_condattr_setclock - get and set the clock selection condition variable attribute

The pthread_condattr_getclock() function shall obtain the value of the clock attribute from the attributes object referenced by attr.

The pthread_condattr_setclock() function shall set the clock attribute in an initialized attributes object referenced by attr. If pthread_condattr_setclock() is called with a clock_id argument that refers to a CPU-time clock, the call shall fail.

The clock attribute is the clock ID of the clock that shall be used to measure the timeout service of pthread_cond_timedwait(). The default value of the clock attribute shall refer to the system clock.

The behavior is undefined if the value specified by the attr argument to pthread_condattr_getclock() or pthread_condattr_setclock() does not refer to an initialized condition variable attributes object.

See https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_condattr_setclock.html

See RFC #51211 for more info.