RFR: 8287281: adjust guarantee in Handshake::execute for the case of target thread being current [v4]

[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 three additional commits since the last revision:

 - do_thread(target) not self
 - Remove checks for is_handshake_for, instead call Handshake::execute
 - Switch order of handshake check

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8992/files
  - new: https://git.openjdk.java.net/jdk/pull/8992/files/b40577f0..bf75d4c8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8992&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8992&range=02-03

  Stats: 15 lines in 3 files changed: 0 ins; 8 del; 7 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