RFR: 8316958: Add test for unstructured locking [v2]

[Roman Kennke]

> In my original lightweight-locking PR, I had a testcase that exercises unstructured locking, which is permitted by the JVM spec, but not generated by javac, and thus not executed anywhere. JITs don't compile unstructured-locked code, therefore any code which attempts this would be slow anyway.
> 
> Testing:
>  - [x] runtime/locking

Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:

  Add Xcomp configuration; remove unneeded code

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15925/files
  - new: https://git.openjdk.org/jdk/pull/15925/files/26eec90d..72f138a7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15925&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15925&range=00-01

  Stats: 9 lines in 1 file changed: 6 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15925.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15925/head:pull/15925

PR: https://git.openjdk.org/jdk/pull/15925