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

This reverts commit 1f502bd9d7d2c1f98ad93a09ffe435e11a95aedd, due to
GCC / MinGW's lack of support for C++11 threading.

It's possible this will go back in after we come up with a
reasonable solution.

llvm-svn: 211401