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

[Andrzej Warzyński via cfe-commits]

https://github.com/banach-space approved this pull request.

LGTM, thanks!

> This has only be tested on x86 Linux.

[nit] Note that the test that you've added will run on any platform. Unless that's referring to something else?

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