Specify -march=nehalem for macOS snapshot builds

Our macOS release builds are limited to Catalina only, and this OS
officially works only on Ivy Bridge (2012) or newer microarchitecture.
Point GCC to target Nehalen (2008) instead - this should cover also all
possible Hackintoshes or virtual machines.

.github/workflows/macos.yml

@@ -52,7 +52,7 @@ jobs:
           echo ::set-env name=VERSION::$VERSION
       - name: Build
         env:
-          FLAGS: -O3 -DNDEBUG -pipe
+          FLAGS: -O3 -DNDEBUG -pipe -march=nehalem
           OPUSFILE_CFLAGS: -I${{ github.workspace }}/contrib/static-opus/include -I${{ github.workspace }}/contrib/static-opus/include/opus
           OPUSFILE_LIBS: ${{ github.workspace }}/contrib/static-opus/lib/libopusfile.a ${{ github.workspace }}/contrib/static-opus/lib/libogg.a ${{ github.workspace }}/contrib/static-opus/lib/libopus.a -lm
         run: |