RFR: 8257020: [JVMCI] enable a JVMCICompiler to specify which GCs it supports [v7]

[Doug Simon]

> A number of jtreg tests require a specific GC. These tests should be ignored when EnableJVMCI is true and the JVMCI compiler does not support the required GC.
> 
> This PR adds `JVMCICompiler.isGCSupported` and makes use of it in `WhiteBox.isGCSupported`.
> 
> Prior to this PR, a test requiring a GC not yet supported by a JVMCI compiler fail as follows:
> Error occurred during initialization of VM
> JVMCI Compiler does not support selected GC: epsilon gc

Doug Simon has updated the pull request incrementally with one additional commit since the last revision:

  Fix @requires vm.jvmci

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1423/files
  - new: https://git.openjdk.java.net/jdk/pull/1423/files/2aa5be51..3848e438

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1423&range=06
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1423&range=05-06

  Stats: 52 lines in 7 files changed: 40 ins; 2 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1423.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1423/head:pull/1423

PR: https://git.openjdk.java.net/jdk/pull/1423