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

[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.

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Commit messages:
 - Disallow handshaking with self

Changes: https://git.openjdk.java.net/jdk/pull/8992/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8992&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287281
  Stats: 8 lines in 3 files changed: 7 ins; 0 del; 1 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