[crac] RFR: Stabilize GHA testsuite

[Radim Vansa]

At this point Github Actions always show some failures; let's ignore platforms that we don't handle for CRaC and some problematic tests in mainline JDK.

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Commit messages:
 - ProblemList test runtime/ClassInitErrors/TestStackOverflowDuringInit.java
 - Exclude s390, ppc64le and riscv64 from build

Changes: https://git.openjdk.org/crac/pull/125/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=125&range=00
  Stats: 20 lines in 2 files changed: 2 ins; 0 del; 18 mod
  Patch: https://git.openjdk.org/crac/pull/125.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/125/head:pull/125

PR: https://git.openjdk.org/crac/pull/125