RFR: 8263856: Github Actions for macos/aarch64 cross-build [v2]

[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.

Anton Kozlov has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Specify Xcode version; don't use --extra-{c,cxx,ld}flags
 - Merge remote-tracking branch 'upstream/jdk/master' into 8263856-gha-macos-aarch64
 - Add macos/aarch64 build workflow

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3270/files
  - new: https://git.openjdk.java.net/jdk/pull/3270/files/3cc31743..215f4dda

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3270&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3270&range=00-01

  Stats: 23610 lines in 431 files changed: 17737 ins; 3221 del; 2652 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