RFR: 8337493: [JVMCI] Number of libgraal threads might be too low

[Raphael Mosaner]

The `-XX:JVMCINativeLibraryThreadFraction` flag defines the ratio between JVMCI threads and C1 threads.
With a default value of 0.33 the number of JVMCI threads is significantly smaller than the number of C2 threads would be.

This can lead to unexpected warmup behavior with `-XX:+UseJVMCICompiler`. This PR changes the default value of `-XX:JVMCINativeLibraryThreadFraction` to yield the same number of JVMCI threads as C2 threads.

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Commit messages:
 - Use the same number of JVMCI threads as C2 threads per default.

Changes: https://git.openjdk.org/jdk/pull/21279/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21279&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337493
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21279.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21279/head:pull/21279

PR: https://git.openjdk.org/jdk/pull/21279