[flang] Avoid dependency of runtime library on pthread for MinGW

When building the Fortran runtime on MinGW, `clock_gettime` is currently used. That function is provided by the `pthread` library on that platform. That means that all programs that link `libFortranRuntime` also require to be linked with `pthread` on that platform.

There is already a code path (for MSVC) that doesn't use `clock_gettime` in the implementation of the Fortran library.
Use the same code path also on MinGW by undefining `CLOCKID`.

Reviewed By: vzakhari

Differential Revision: https://reviews.llvm.org/D149051