RFR: 8309136: [JVMCI] add -XX:+UseGraalJIT flag

[Doug Simon]

Use of the Graal-based JIT in OpenJDK currently requires the following flag: `-XX:+EnableJVMCIProduct`

This has no direct association with Graal. If the JDK image happens to include a non-Graal JVMCI implementation, it will be automatically selected. This would come as a surprise to users who equate JVMCI with Graal.

This PR introduces a new flag, `-XX:+UseGraalJIT` to address these shortcomings. The VM fails fast at startup if there is a non-Graal JVMCI implementation or no JVMCI implementation in the JDK image.

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Commit messages:
 - add UseGraalJIT VM flag

Changes: https://git.openjdk.org/jdk/pull/14231/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14231&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309136
  Stats: 53 lines in 4 files changed: 36 ins; 0 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/14231.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14231/head:pull/14231

PR: https://git.openjdk.org/jdk/pull/14231