RFR: 8358169: Shenandoah/JVMCI: Export GC state constants

[Roman Kennke]

We need the GC state enum constants available in JVMCI.

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Commit messages:
 - 8358169: Shenandoah/JVMCI: Export GC state constants

Changes: https://git.openjdk.org/jdk/pull/25552/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25552&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358169
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/25552.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25552/head:pull/25552

PR: https://git.openjdk.org/jdk/pull/25552