MFC r367742-r367743: _umtx_op: documentation and compat32 fix

r367742:
_umtx_op: document UMTX_OP_SEM2_WAIT copyout behavior

This clever technique to get a time remaining back was added to support
sem_clockwait_np.

Reviewed by: kib, vangyzen
Differential Revision: https://reviews.freebsd.org/D27160

r367743:
_umtx_op: fix a compat32 bug in UMTX_OP_NWAKE_PRIVATE

Specifically, if we're waking up some value n > BATCH_SIZE, then the
copyin(9) is wrong on the second iteration due to upp being the wrong type.
upp is currently a uint32_t**, so upp + pos advances it by twice as many
elements as it should (host pointer size vs. compat32 pointer size).

Fix it by just making upp a uint32_t*; it's still technically a double
pointer, but the distinction doesn't matter all that much here since we're
just doing arithmetic on it.

Add a test case that demonstrates the problem, placed with the libthr tests
since one messing with _umtx_op should be running these tests. Running under
compat32, the new test case will hang as threads after the first 128 get
missed in the wake. it's not immediately clear how to hit it in practice,
since pthread_cond_broadcast() uses a smaller (sleepq batch?) size observed
to be around ~50 -- I did not spend much time digging into it.

The uintptr_t change makes no functional difference, but i've tossed it in
since it's more accurate (semantically).