RFR: 8341260: Add Float16 to jdk.incubator.vector

[Joe Darcy]

Port of Float16 from java.lang in the lworld+fp16 branch to jdk.incubabor.vector.

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

Commit messages:
 - Update with changes from lworld+fp16 Float16.
 - Merge branch 'master' into JDK-8341260
 - Add support for BigDecimal -> Float16 conversion.
 - JDK-8341260: Add Float16 to jdk.incubator.vector

Changes: https://git.openjdk.org/jdk/pull/21574/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21574&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341260
  Stats: 2355 lines in 4 files changed: 2350 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/21574.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21574/head:pull/21574

PR: https://git.openjdk.org/jdk/pull/21574