[compiler-rt] Make SANITIZER_MIN_OSX_VERSION a cache variable (PR #74394)

[Julian Lettner via llvm-commits]

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

Let me summarize my understanding: the intention here is to promote `SANITIZER_MIN_OSX_VERSION` to a CMAKE cache variable.  We don't change any behavior or default.

LGTM, thanks!  

@rsundahl @thetruestblue
At some point we should think about bumping the minimal sanitizer target enabling some cleanups.

https://github.com/llvm/llvm-project/pull/74394