Replace Execution Engine's mutex with std::recursive_mutex.

This change has a bit of a trickle down effect due to the fact that
there are a number of derived implementations of ExecutionEngine,
and that the mutex is not tightly encapsulated so is used by other
classes directly.

Reviewed by: rnk

Differential Revision: http://reviews.llvm.org/D4196

llvm-svn: 211214