Profiler can deadlock on OS X

[Keno]

If we happen to stop a thread while it is trying to register a dynamic fde, the profiler thread will dead lock trying to acquire a lock held by the blocked thread.

[vtjnash]

I started some work to address this on my threading branch

[Keno]

Can we move that work onto master?

[yuyichao]

@vtjnash Is this fixed? (Or is this still not due to one of your todo's in signals-mach.c?)

[vtjnash]

no, i think keno wanted us to add a guard around calls to __register_frame that stops us from trying to compute a backtrace then