Rename Makefile.osx to Makefile.macos, make it arch-neutral, add a macos-universal variant that explicitly does x86_64 + arm64 build

.github/workflows/macos-ios.yml

@@ -15,8 +15,12 @@ jobs:
     - uses: actions/checkout@v2
     - name: deps
       run: brew install libsndfile vamp-plugin-sdk
-    - name: make macos
-      run: make -f Makefile.osx clean library program vamp
+    - name: make macos universal library
+      run: make -f Makefile.macos-universal clean library
+    - name: make macos native binaries
+      # (these depend on things installed from Homebrew, which I think
+      # doesn't install universal anything, so let's just make them native)
+      run: make -f Makefile.macos program vamp
     - name: make ios
       run: make -f Makefile.ios clean all