Fix a race between lldb's packet timeout and the profile thread's usleep.

The debugserver profile thread used to suspend itself between samples with
a usleep.  When you detach or kill, MachProcess::Clear would delay replying
to the incoming packet until pthread_join of the profile thread returned.
If you are unlucky or the suspend delay is long, it could take longer than
the packet timeout for pthread_join to return.  Then you would get an error
about detach not succeeding from lldb - even though in fact the detach was
successful...

I replaced the usleep with PThreadEvents entity.  Then we just call a timed
WaitForEventBits, and when debugserver wants to stop the profile thread, it
can set the event bit, and the sleep will exit immediately.

Differential Revision: https://reviews.llvm.org/D75004