[PR] Instrumentation: Disable signals on mutex lock

Summary:
When indirect call is instrmented it locks SimpleHashTable's mutex on get() call.
If while locked we we receive a signal and signal handler also will call
indirect function we will end up with deadlock.

PR facebookincubator/BOLT#167

Vladislav Khmelevsky,
Advanced Software Technology Lab, Huawei

(cherry picked from FBD28909921)