RFR: 8311556: GetThreadLocalStorage not working for vthreads mounted during JVMTI attach

[Serguei Spitsyn]

This is an issue with a dynamic load of a JVMTI agent into running VM.
The `VM_SetNotifyJvmtiEventsMode` enabling operation makes a call to the function `count_transitions_and_correct_jvmti_thread_states()`. This function in its turn make a call to the function `correct_jvmti_thread_state()`. But it does it conditionally, only if the field `_whitebox_used` is `true`.
The test provided in the bug report showed that it has to be called unconditionally as the assumption that it is only needed on the subsequent `notifyJvmti` enabling is incorrect.

Then the field `_whitebox_used` is not needed anymore and removed in this fix.
Some obsolete comments are removed or updated.

New test is added: `test/hotspot/jtreg/serviceability/jvmti/vthread/VThreadTLSTest`.
It is failed without the fix and passed with the fix. 

Testing:
  - ran new test `test/hotspot/jtreg/serviceability/jvmti/vthread/VThreadTLSTest`
  - mach5 tiers 1-5 are good

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Commit messages:
 - fix traling spaces and remove unneeded empty lines in new test
 - 8311556: GetThreadLocalStorage not working for vthreads mounted during JVMTI attach

Changes: https://git.openjdk.org/jdk/pull/14842/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14842&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311556
  Stats: 198 lines in 3 files changed: 183 ins; 13 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14842.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14842/head:pull/14842

PR: https://git.openjdk.org/jdk/pull/14842