Integrated: ContMirror::allocate_stack_chunk was using the wrong thread for allocation

[Coleen Phillimore]

Pull request because something wants a password when I push this tiny change directly.

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Commit messages:
 - ContMirror::allocate_stack_chunk was using the wrong thread for allocation.

Changes: https://git.openjdk.java.net/loom/pull/51/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=51&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/loom/pull/51.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/51/head:pull/51

PR: https://git.openjdk.java.net/loom/pull/51