[clang] [flang] [flang] Link to libatomic with openmp and rtlib=libgcc (PR #112202)

[Michael Klemm via cfe-commits]

https://github.com/mjklemm approved this pull request.

LGTM, I think this simplifies using the compiler a bit when it comes to OpenMP and more complex `atomic` regions.

https://github.com/llvm/llvm-project/pull/112202