[clang] Bring --gcc-triple to flang (PR #165886)

[Tulio Magno Quites Machado Filho via cfe-commits]

https://github.com/tuliom requested changes to this pull request.

I believe it's important to add a test.
Similar to `clang/test/Driver/gcc-triple.cpp`.

https://github.com/llvm/llvm-project/pull/165886