hang during detach due to xl8 of signal arriving while suspended

This is seen in a 3rd-party app. It is very similar to what happened here: https://github.com/DynamoRIO/dynamorio/issues/2779#issuecomment-391195431

A thread is suspended and it relaxes its signal mask. A signal arrives and is set for receive_now due to the #2659 (closed) check to deliver while at a client safe spot. The subsequent xl8 wants thread_initexit_lock which deadlocks.

The #2659 (closed) check should rule out a suspended thread.