RFR: 8287281: adjust guarantee in Handshake::execute for the case of target thread being current [v2]

[Johan Sjölén]

> Please review this PR for fixing JDK-8287281.
> 
> I chose a different solution than the one suggested. Looking at all callers of `Handshake::execute`, it seems that only one depends on `target == current`. The rest special case that by calling `is_handshake_safe_for` and `do_thread` directly.  I converted the only instance of `Handshake::execute` with `target == current` to just directly call `do_thread`.
> 
> Furthermore we now explicitly check for this case in `Handshake::execute` with an assert and document that this should not be done.
> 
> Finally:  Should `VirtualThreadGetThreadClosure` and its `do_thread()` body be inlined instead? We can do this in this PR, imho, but I'm hoping to get some input on this.
> 
> 
> Currently running tier1-5 to check if I'm missing something.

Johan Sjölén has updated the pull request incrementally with one additional commit since the last revision:

  Call do_thread if handshaking with current thread as target

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8992/files
  - new: https://git.openjdk.java.net/jdk/pull/8992/files/14bee58c..b7b4c086

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8992&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8992&range=00-01

  Stats: 9 lines in 3 files changed: 2 ins; 5 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8992.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8992/head:pull/8992

PR: https://git.openjdk.java.net/jdk/pull/8992