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

[Chris Apple via llvm-commits]

cjappl wrote:

> 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.

Your understanding is correct! No changes in behavior, just allowing people to tweak it easier.

> At some point we should think about bumping the minimal sanitizer target enabling some cleanups.

One of the main reasons behind me putting up this review is I found a handful of things that would be cleaned up (allowing sanitizers to hook into calls introduced in newer MacOS versions). Once this is merged, I'm going to put up a few more in this series based on what I found! I'll make sure to add you to those reviews to get your thoughts @yln . 

Thanks for the review.

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