Fix issue where MACOSX_VERSION_MIN_FLAG was not set on subsequent runs of...

Fix issue where MACOSX_VERSION_MIN_FLAG was not set on subsequent runs of CMake in compiler-rt (#87580)

As discussed here:

https://github.com/llvm/llvm-project/pull/74394#issuecomment-2035264683

An unintentional change of behavior was introduced in #74394 

This code introduced in #74394 :

The first time through
* SANITIZER_MIN_OSX_VERSION is not set
* parse -mmacosx-version-min and set MACOSX_VERSION_MIN_FLAG
* Set and cache SANITIZER_MIN_OSX_VERSION

Subsequent times through:
* SANITIZER_MIN_OSX_VERSION is cached 
* (BUG!!) you don't parse -mmacosx-version-min, and don't set
MACOSX_VERSION_MIN_FLAG


MACOSX_VERSION_MIN_FLAG is used later in the file on this line:

https://github.com/llvm/llvm-project/blob/63c925ca808f216f805b76873743450456e350f2/compiler-rt/cmake/config-ix.cmake#L517


Hoisting this assignment outside the if block returns us to the previous
behavior before this commit, while maintaining the flexibility
introduced with the cache variable