[flang][runtime] Remove dependency on C++ <mutex> on Windows

The implementation of the Lock class on Windows currently uses C++
mutexes. That introduces a dependency on the C++ runtime on that
platform.

Use a Windows CriticalSection instead of a std::mutex to avoid that
dependency.

This works for me with MinGW (tested in a CLANG64 environment of MSYS2).

See also D126291.

Differential Revision: https://reviews.llvm.org/D127316