Add config option and compiler support to reuse across OS's (#42693)

* Allow compilers to function across compatible OS's
* Add documentation in the default yaml

Co-authored-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>
Co-authored-by: Gregory Becker <becker33@llnl.gov>

etc/spack/defaults/concretizer.yaml

@@ -42,3 +42,8 @@ concretizer:
     # "minimal": allows the duplication of 'build-tools' nodes only (e.g. py-setuptools, cmake etc.)
     # "full" (experimental): allows separation of the entire build-tool stack (e.g. the entire "cmake" subDAG)
     strategy: minimal
+  # Option to specify compatiblity between operating systems for reuse of compilers and packages
+  # Specified as a key: [list] where the key is the os that is being targeted, and the list contains the OS's 
+  # it can reuse. Note this is a directional compatibility so mutual compatibility between two OS's 
+  # requires two entries i.e. os_compatible: {sonoma: [monterey], monterey: [sonoma]}
+  os_compatible: {}