[jdk21u-dev] RFR: 8342988: GHA: Build JTReg in single step

[Aleksey Shipilev]

Makes sure GHA is more reliable: we do not risk failures when building jtreg multiple times, and it also allows us to compile it once on x86, without bothering with different arch JDKs.

The original PR does not apply cleanly, because Alpine support added by [JDK-8282944](https://bugs.openjdk.org/browse/JDK-8282944) is not in JDK 21. I have resolved the block by hand.

Additional testing:
 - [ ] GHA

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Commit messages:
 - Backport 88dc655a6d6cfc71c467405b62bd35beeed7794f

Changes: https://git.openjdk.org/jdk21u-dev/pull/1269/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1269&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342988
  Stats: 138 lines in 3 files changed: 81 ins; 26 del; 31 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1269.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1269/head:pull/1269

PR: https://git.openjdk.org/jdk21u-dev/pull/1269