[llvm] [Support/rpmalloc] Updated fake atomics with Interlocked functions (PR #148303)

[Alexandre Ganea via llvm-commits]

aganea wrote:

I'd like to see this merged upstream rpmalloc before merging it here. @slydiman Can you send a PR here please first? https://github.com/mjansson/rpmalloc/pulls

You might also want to experiment with the compiler flag [`/experimental:c11atomics`](https://devblogs.microsoft.com/cppblog/c11-atomics-in-visual-studio-2022-version-17-5-preview-2/) in which case the codepath would be the same as for Clang. I suppose we could add that flag only files in `llvm/lib/Support/rpmalloc/`.

https://github.com/llvm/llvm-project/pull/148303