[lworld+fp16] RFR: 8341639: Add float16ToShortBits(), signum(), copySign() to Float16

[Raffaello Giulietti]

These common operations are currently lacking for Float16, and should be added.

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Commit messages:
 - 8341639: Add float16ToShortBits(), signum(), copySign() to Float16

Changes: https://git.openjdk.org/valhalla/pull/1268/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1268&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341639
  Stats: 178 lines in 3 files changed: 174 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/valhalla/pull/1268.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1268/head:pull/1268

PR: https://git.openjdk.org/valhalla/pull/1268