RFR: 8257020: [JVMCI] enable a JVMCICompiler to specify which GCs it supports [v6]

[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:

  added jvmci tag to new warning and improved its message

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1423/files
  - new: https://git.openjdk.java.net/jdk/pull/1423/files/0ac0959e..2aa5be51

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1423&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1423&range=04-05

  Stats: 2 lines in 2 files changed: 1 ins; 0 del; 1 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