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

[Raffaello Giulietti]

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

-------------

Commit messages:
 - 8339252: Add method float16Value() to BigInteger and BigDecimal
 - Added valueOf(BigDecimal).
 - Merge lworld
 - 8334432: Refine Float16.fma
 - 8337605: Update description of constants in Float16
 - 8338102: x86 backend support for newly added Float16 intrinsics.
 - 8336406: Add support for FP16 binary operations
 - 8330021: AArch64: Add backend support for FP16 add operation
 - 8333852: Allow flat array layout for value based classes.
 - 8329817: Augment prototype Float16 class
 - ... and 5 more: https://git.openjdk.org/valhalla/compare/e19d6796...01418c65

Changes: https://git.openjdk.org/valhalla/pull/1228/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1228&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339252
  Stats: 5810 lines in 78 files changed: 4998 ins; 4 del; 808 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