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

[Chris Apple via llvm-commits]

cjappl wrote:

Pinging either:

@rsundahl for a review

Or one of my previous reviewers with write access for a merge, if appropriate.

@petrhosek @yln @vitalybuka

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