RFR: 8308000: add PopFrame support for virtual threads [v4]

[Serguei Spitsyn]

> This enhancement adds `PopFrame` support for virtual threads. The spec defines minimal support that the JVMTI `PopFrame` can be used for a virtual thread suspended an an event. 
> Actually, the `PopFrame` can supports suspended and mounted virtual threads. 
> 
> CSR (approved): https://bugs.openjdk.org/browse/JDK-8308001: add PopFrame support for virtual threads
> 
> Testing:
> New test was developed: `serviceability/vthread/PopFrameTest`.
> Submitted mach5 tiers 1-6 are good.
> TBD: rerun mach5 tiers 1-6 at the end of review again if necessary.

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

  removed unused variables in libPopFrameTest.cpp

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14002/files
  - new: https://git.openjdk.org/jdk/pull/14002/files/31e00bd9..8f58f6d2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14002&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14002&range=02-03

  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14002.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14002/head:pull/14002

PR: https://git.openjdk.org/jdk/pull/14002