[llvm] [SystemZ] Add proper mcount handling (PR #135767)

[Dominik Steenken via llvm-commits]

dominik-steenken wrote:

@RKSimon I'm not exactly sure what i'm looking at with this sanitizer test fail. I'm currently trying to reproduce it locally. However, looking at that buildbot, it seems [the next build](https://lab.llvm.org/buildbot/#/builders/66/builds/13406) is fine, so i'm cautiously optimistic that this was some sort of transient failure.

https://github.com/llvm/llvm-project/pull/135767