Allow loading _cinderx on macOS

Summary:
It won't work for most users because of aarch64, but at least it'll help us
experiment.

Reviewed By: yoney

Differential Revision: D84923694

fbshipit-source-id: 33a9f18ee7aaeff762831030adab08875fab1af5

Author: alexmalyshev

cinderx/PythonLib/cinderx/__init__.py

@@ -32,7 +32,7 @@ def is_supported_runtime() -> bool:
     native extension.
     """
 
-    if sys.platform != "linux":
+    if sys.platform not in ("darwin", "linux"):
         return False
 
     version = (sys.version_info.major, sys.version_info.minor)