Default to android for local builds on macOS

Summary: Buck uses the host platform as the target platform when otherwise unspecified. As a side-effect, local builds on macOS default to 'macos' platform monikers without explicit modifiers, when the desired default has historically been 'android'. This fixes by using an already-in-use required constraint setting value for 'macos' platform targeting, allowing the 'android' fallback to work as expected.

Reviewed By: robhogan

Differential Revision: D68902337

fbshipit-source-id: e48c72371187bd16b9daf10a92c3d0d95e01daf0

prelude/js/js.bzl

@@ -18,9 +18,9 @@ def _select_platform():
         "DEFAULT": select({
             "DEFAULT": "android",
             "config//os/constraints:iphoneos": "ios",
-            "config//os/constraints:macos": "macos",
             "config//os/constraints:windows": "windows",
         }),
+        "config//react-native:macos": "macos",
         # TODO(T210407097): Remove after deleting //third-party/microsoft-fork-of-react-native
         "fbsource//tools/build_defs/js/config:macos_legacy": "macos_legacy",
         "fbsource//tools/build_defs/js/config:platform_override_android": "android",