RFR: 8357370: Export supported GCs in JVMCI [v3]

[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

Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:

  Align most trailing \s

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Changes:
  - all: https://git.openjdk.org/jdk/pull/25325/files
  - new: https://git.openjdk.org/jdk/pull/25325/files/321a0940..16d82e7b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25325&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25325&range=01-02

  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 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