RFR: 8308000: add PopFrame support for virtual threads [v8]

[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:

  PopFrameTest improvements

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14002/files
  - new: https://git.openjdk.org/jdk/pull/14002/files/b860b1d9..d42872f5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14002&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14002&range=06-07

  Stats: 26 lines in 3 files changed: 11 ins; 4 del; 11 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