RFR: 8340576: Some JVMCI flags are inconsistent [v2]

[Doug Simon]

> This PR replaces some uses of `UseJVMCICompiler` with `EnableJVMCI` so that JVMCI code paths are taken when JVMCI is only used for non-CompilerBroker compilations.

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

  clarified doc for EnableJVMCI and UseJVMCINativeLibrary

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21120/files
  - new: https://git.openjdk.org/jdk/pull/21120/files/fcc3ece0..e26e68d9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21120&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21120&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/21120.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21120/head:pull/21120

PR: https://git.openjdk.org/jdk/pull/21120