[lworld+fp16] RFR: 8339252: Add method float16Value() to BigInteger and BigDecimal [v3]

[Raffaello Giulietti]

> This patch adds `float16Value()` to `BigInteger` and `BigDecimal`, as well as `Float16.valueOf(BigDecimal)`.

Raffaello Giulietti has updated the pull request incrementally with one additional commit since the last revision:

  More clarification in a comment.

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1228/files
  - new: https://git.openjdk.org/valhalla/pull/1228/files/01418c65..0d76e859

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1228&range=02
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1228&range=01-02

  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/valhalla/pull/1228.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1228/head:pull/1228

PR: https://git.openjdk.org/valhalla/pull/1228