RFR: 7537: build script assumes that Darwin always targets x86_64

[Johannes Bechberger]

Tested on Mac M1.

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Commit messages:
 - Add aarch64 support to build.sh

Changes: https://git.openjdk.java.net/jmc/pull/366/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=366&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7537
  Stats: 13 lines in 2 files changed: 7 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jmc/pull/366.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/366/head:pull/366

PR: https://git.openjdk.java.net/jmc/pull/366