libc/libpthread: Add clock_getcpuclockid() and pthread_getcpuclockid().

* Adjust clock_gettime() and clock_getres() to accept values obtained
this way.

* Also set _POSIX_CPUTIME and _POSIX_THREAD

libc/libpthread: Add clock_getcpuclockid() and pthread_getcpuclockid().

* Adjust clock_gettime() and clock_getres() to accept values obtained
this way.

* Also set _POSIX_CPUTIME and _POSIX_THREAD_CPUTIME, although we should
really support values obtained by these functions in clock_settime()
too.

Based on and taken from FreeBSD's code.

Reviewed-by: sephe

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