Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Implement syscall clock_getcpuclockid2, so we can get a clock id
for process, thread or others we want to support.
Use the syscall to implement POSIX API clock_getcpuclock and
pthread_getcpuclockid.

PR: 168417

Add POSIX pthread API pthread_getcpuclockid() to get a thread's cpu
time clock id.