RFR: 8296323: JVMTI can_support_virtual_threads not available for agents loaded into running VM [v6]

[Serguei Spitsyn]

> The can_support_virtual_thread was initially implemented as an onload capability.
> It is why this capability does not work for the agents loaded into running VM.
> The fix is to move it from `onload` to `always`capabilities list.
> 
> Testing:
> New test is added: VirtualStartThreadTest.
> TBD: mach5 jvmti, jdi and tier1-6 tests.

Serguei Spitsyn has updated the pull request incrementally with one additional commit since the last revision:

  removed thread->vthread() != NULL from  JvmtiVirtualThreadEventMark constructor

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11246/files
  - new: https://git.openjdk.org/jdk/pull/11246/files/7c9c3b5b..1a8f9810

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11246&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11246&range=04-05

  Stats: 5 lines in 1 file changed: 0 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11246.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11246/head:pull/11246

PR: https://git.openjdk.org/jdk/pull/11246