Allow the internal-state-thread free access to the TargetAPI mutex.

It is always doing work on behalf of another thread that presumably
has the mutex, so if it is calling SB API's it should have free access
to the mutex.  This is the same decision as we made earlier with the
process RunLock.

Differential Revision: https://reviews.llvm.org/D68174

llvm-svn: 373280