RFR: 8357370: Export supported GCs in JVMCI

[Roman Kennke]

I need a way to detect in JVMCI if Shenandoah GC is supported (that is, built-in) by HotSpot. I need it for Shenandoah, because some vendors don't build it, but for cleanliness the relevant preprocessor constants should be exported for all GCs.

Testing:
 - [x] build/test https://github.com/oracle/graal/pull/10904

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Commit messages:
 - 8357370: Export supported GCs in JVMCI

Changes: https://git.openjdk.org/jdk/pull/25325/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25325&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357370
  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/25325.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25325/head:pull/25325

PR: https://git.openjdk.org/jdk/pull/25325