[clang] [Flang][driver] Do not emit -latomic on link line on Windows (PR #164648)

[David Truby via cfe-commits]

DavidTruby wrote:

Am I right in thinking this is only for MinGW targets and not msvcrt ones? I.e. with a triple like `x86_64-pc-windows-gnu` instead of `x86_64-pc-windows-msvc`; I think in the latter we don't add this already.
If so, I think it looks fine but I wonder where you're going to get the atomic symbols from in that case? Will programs that need them that use this triple just not work? 

https://github.com/llvm/llvm-project/pull/164648