[jdk17u-dev] RFR: 8286490: JvmtiEventControllerPrivate::set_event_callbacks CLEARING_MASK computation is incorrect

[Roman Marchenko]

The backport isn't clean.

1. Resolved a conflict in `src/hotspot/share/prims/jvmtiEventController.cpp` with an empty line.
2. Removed files as VThreads are not implemented in jdk17:
      - `test/hotspot/jtreg/serviceability/jvmti/vthread/VThreadTest/libVThreadTest.cpp` 
      - `test/hotspot/jtreg/serviceability/jvmti/vthread/VThreadTest/VThreadTest.java`
3. Added new test:
      - `test/hotspot/jtreg/vmTestbase/nsk/jvmti/unit/extcallback/Test.java`
      - `test/hotspot/jtreg/vmTestbase/nsk/jvmti/unit/extcallback/libextcallback.c`
      - `test/hotspot/jtreg/vmTestbase/nsk/jvmti/unit/extcallback/loadClass/Test2.java`

Actually, backporting the fix only.
The new test fails without the fix, and successfully passes with the fix.

-------------

Commit messages:
 - Added test for the fix.
 - 8286490: JvmtiEventControllerPrivate::set_event_callbacks CLEARING_MASK computation is incorrect

Changes: https://git.openjdk.org/jdk17u-dev/pull/2276/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=2276&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8286490
  Stats: 166 lines in 4 files changed: 164 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/2276.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/2276/head:pull/2276

PR: https://git.openjdk.org/jdk17u-dev/pull/2276