Add a PTHREADPOOL_FLAG_DONT_SPIN_WORKERS flag to:

* Wait on a `futex` or `cond_var`, or
* Returns threads to an executor immediately,
instead of spinning for a limited number of iterations while waiting for work.

This flag should replace `PTHREADPOOL_FLAG_YIELD_WORKERS`, which is currently the default behaviour.

PiperOrigin-RevId: 810004216

Author: gonnet