RFR: Loom preempt3 [v3]

[Coleen Phillimore]

> This is my cleaned up version of preemption where one thread suspends another, walks the stack and allocates the frozen frame. There are some asserts to verify this preempt situation and the jump in handle_special_runtime_exit_condition might need to be moved somewhere safer.  Robbin wasn't happy with the fact that while suspended, another thread can handshake this thread and modify the stack, even though the loom code doesn't do that.  I think this is better for now though and hasn't significantly changed in a while, so I'd like to check this in for now.

Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:

  Better handling boolean for preempt

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Changes:
  - all: https://git.openjdk.java.net/loom/pull/48/files
  - new: https://git.openjdk.java.net/loom/pull/48/files/ec351957..14fda9ce

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=loom&pr=48&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=loom&pr=48&range=01-02

  Stats: 12 lines in 1 file changed: 0 ins; 3 del; 9 mod
  Patch: https://git.openjdk.java.net/loom/pull/48.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/48/head:pull/48

PR: https://git.openjdk.java.net/loom/pull/48