RFR: JDK-8289551: Conversions between bit representations of half precision values and floats

[Joe Darcy]

Initial implementation.

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Commit messages:
 - Further refine spec.
 - Refine spec and tests.
 - Fix implementation thresholds; refine tests.
 - Initial implementation of float -> binary16 round-to-nearest with tests.
 - Implement review feedback from John Rose.
 - Merge branch 'master' into JDK-8289551
 - Merge branch 'master' into JDK-8289551
 - Appease jcheck.
 - Add cardinal value tests.
 - JDK-8289551: Conversions between bit representations of half precision values and floats

Changes: https://git.openjdk.org/jdk/pull/9422/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9422&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289551
  Stats: 464 lines in 2 files changed: 464 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9422.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9422/head:pull/9422

PR: https://git.openjdk.org/jdk/pull/9422