RFR: 8311946: add support for libgraal specific jtreg tests

[Doug Simon]

This PR adds support for a `vm.libgraal.enabled` property that can be used with the jtreg `@requires` tag to determine if the VM under test is using libgraal as the JIT.

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Commit messages:
 - add support for vm.libgraal.enabled property

Changes: https://git.openjdk.org/jdk/pull/14851/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14851&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311946
  Stats: 23 lines in 3 files changed: 23 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14851.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14851/head:pull/14851

PR: https://git.openjdk.org/jdk/pull/14851