[flang] Use std::mutex instead of pthreads (flang-compiler/f18#1006)

* Use std::mutex for portability.

Original-commit: flang-compiler/f18@a9a725eb9d2c39a203f4b4f55f1e03c5e17f6cf7
Reviewed-on: https://github.com/flang-compiler/f18/pull/1006