RFR: 8263856: Github Actions for macos/aarch64 cross-build

[Anton Kozlov]

Please review a change that adds cross-compiled macos/aarch64 platform build into Github Actions test workflow.

The steps are basically copy-paste of macos/x86-64 build with the necessary platform name adjustments. A default XCode is also used, as macos/aarch64 build requires XCode 12+.

I've checked the produced build (can be found in https://github.com/AntonKozlov/jdk/actions/runs/667527036), it starts on the actual aarch64 machine.

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Commit messages:
 - Add macos/aarch64 build workflow

Changes: https://git.openjdk.java.net/jdk/pull/3270/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3270&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263856
  Stats: 107 lines in 1 file changed: 107 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3270.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3270/head:pull/3270

PR: https://git.openjdk.java.net/jdk/pull/3270