RFR: 8340733: Add scope for relaxing constraint on JavaCalls from CompilerThread [v6]

[Tomáš Zezula]

> [JDK-8318694](https://bugs.openjdk.org/browse/JDK-8318694) limited the ability for JVMCI CompilerThreads to make Java upcalls. This is to mitigate against deadlock when an upcall does class loading. Class loading can easily create deadlock situations in -Xcomp or -Xbatch mode.
> 
> However, for Truffle, upcalls are unavoidable if Truffle partial evaluation occurs as part of JIT compilation inlining. This occurs when the Graal inliner sees a constant Truffle AST node which allows a Truffle-specific inlining extension to perform Truffle partial evaluation (PE) on the constant. Such PE involves upcalls to the Truffle runtime (running in Java).
> 
> This PR provides the escape hatch such that Truffle specific logic can put a compiler thread into "allow Java upcall" mode during the scope of the Truffle logic.

Tomáš Zezula has updated the pull request incrementally with one additional commit since the last revision:

  Improved a comment in CompilerThread.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21285/files
  - new: https://git.openjdk.org/jdk/pull/21285/files/e07d4448..7e0f1a42

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21285&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21285&range=04-05

  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21285.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21285/head:pull/21285

PR: https://git.openjdk.org/jdk/pull/21285