RFR: 8256747: GitHub Actions: decouple the hotspot build-only jobs from Linux x64 testing

[Robin Westberg]

Currently Linux x64 testing in GitHub Actions depends on a few non-relevant hotspot build-only jobs (such as zero) that prevents testing from being run if those build were to fail. As the tests only require the x64 release and debug builds to run and provide interesting feedback, we should separate these other builds into a different job.

To reduce duplicated steps, all the existing hotspot-only builds have been consolidated into a single job.

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

Commit messages:
 - Separate hotspot-only builds into a separate job

Changes: https://git.openjdk.java.net/jdk/pull/1350/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1350&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256747
  Stats: 517 lines in 1 file changed: 32 ins; 438 del; 47 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1350.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1350/head:pull/1350

PR: https://git.openjdk.java.net/jdk/pull/1350