[flang] [llvm] [openmp] [flang][OpenMP] Compile proper `omp_lib.mod` from the `openmp/src/include` sources (PR #80874)

[Michael Klemm via llvm-commits]

mjklemm wrote:

Alrighty, I have also been able to test this on Windows and everything seems to be working fine there, too.

Anyone objecting to merge this?  If not, please review and approve. :-)

https://github.com/llvm/llvm-project/pull/80874