[flang] [clang] [flang][Driver] Support -pthread in the frontend (PR #77360)

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

Thanks for the input @ergawy !

> That said, what @tarunprabhu mentioned about OpenMPI wrappers adding the flag is indeed correct as mentioned by Brian Cornille in the last flang bi-weekly. So adding the flag would indeed make sense even if it is redundant for the GNU toolchain but it would be consistent with clang. However, coming up with a failing test will prove more difficult that it initially seems.

Could you take a look at https://github.com/llvm/llvm-project/pull/77135 and see whether `-gpulibc` could be helpful for testing?

https://github.com/llvm/llvm-project/pull/77360