[lworld+fp16] RFR: 8339599: Add nextUp(), nextDown(), scalb() to Float16

[Raffaello Giulietti]

Adding these to Float16 rather than Math|StrictMath.

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Commit messages:
 - 8339599: Add nextUp(), nextDown(), scalb() to Float16

Changes: https://git.openjdk.org/valhalla/pull/1238/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1238&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339599
  Stats: 496 lines in 4 files changed: 481 ins; 8 del; 7 mod
  Patch: https://git.openjdk.org/valhalla/pull/1238.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1238/head:pull/1238

PR: https://git.openjdk.org/valhalla/pull/1238