Instrument pthread_create and pthread_join
Instrument pthread_create and pthread_join

instead of instrumenting entry and exits of thread functions for two
reasons:
1. IdentifyThreadFuncs can be very imprecise, which causes a thread to
call EnterThread or ExitThread multiple times.
2. Sometimes a thread function is called as a regular function. We want
to avoid calling EnterThread or ExitThread in that case.

wujingyue/loom,wujingyue/loom,wujingyue/loom