RFR: Add preempt logging to find bug.

[Coleen Phillimore]

I made the logging product level and added the preempt tag for preemption, in order to find out why the Preempt.java test is failing a lot.  It seems to only be failing in the interpreter test cases.  Anyway, if performance is a concern with preemption, we can move this back to log_develop_trace.

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Commit messages:
 - Add preempt logging to find bug.

Changes: https://git.openjdk.java.net/loom/pull/56/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=56&range=00
  Stats: 77 lines in 3 files changed: 37 ins; 8 del; 32 mod
  Patch: https://git.openjdk.java.net/loom/pull/56.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/56/head:pull/56

PR: https://git.openjdk.java.net/loom/pull/56