[jdk17u-dev] RFR: 8338286: GHA: Demote x86_32 to hotspot build only

[Aleksey Shipilev]

This should make GHAs cleaner and more efficient for future backports.

This backport is a bit unclean: there was a minor conflict in `default` line, which does not include some platform builds that are present past JDK 17.

Additional testing:
 - [ ] GHA

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Commit messages:
 - Backport da7311bbe37c2b9632b117d52a77c659047820b7

Changes: https://git.openjdk.org/jdk17u-dev/pull/2816/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=2816&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338286
  Stats: 19 lines in 1 file changed: 1 ins; 11 del; 7 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/2816.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/2816/head:pull/2816

PR: https://git.openjdk.org/jdk17u-dev/pull/2816