Support building on Big Sur

The Kokoro big-sur machines only have Xcode 12.5.1, while the
catalina machines have 12.2 and 12.4.

https://g3doc.corp.google.com/devtools/kokoro/g3doc/development/macos_external_catalina_config.md

https://g3doc.corp.google.com/devtools/kokoro/g3doc/development/macos_external_big_sur_config.md

By building on big-sur, it should be possible to remove the
0001-Enable-arm64-builds.patch, and we also wouldn't have to
change the default Xcode.

Test: kokoro presubmit
Change-Id: Ia8e98d47de7e0c248484d2285fb20c9062b8a546
1 file changed