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

As discussed here:

#74394 (comment)

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

Author: cjappl

compiler-rt/cmake/config-ix.cmake

@@ -463,9 +463,11 @@ if(APPLE)
 
   set(DEFAULT_SANITIZER_MIN_OSX_VERSION 10.13)
   set(DARWIN_osx_MIN_VER_FLAG "-mmacosx-version-min")
+
+  string(REGEX MATCH "${DARWIN_osx_MIN_VER_FLAG}=([.0-9]+)"
+         MACOSX_VERSION_MIN_FLAG "${CMAKE_CXX_FLAGS}")
+
   if(NOT SANITIZER_MIN_OSX_VERSION)
-    string(REGEX MATCH "${DARWIN_osx_MIN_VER_FLAG}=([.0-9]+)"
-           MACOSX_VERSION_MIN_FLAG "${CMAKE_CXX_FLAGS}")
     if(MACOSX_VERSION_MIN_FLAG)
       set(MIN_OSX_VERSION "${CMAKE_MATCH_1}")
     elseif(CMAKE_OSX_DEPLOYMENT_TARGET)