[vectorIntrinsics+fp16] RFR: 8290204: FP16 initial backend implementation

[Smita Kamath]

Initial backend implementation for enabling FP16.

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

Commit messages:
 - Initial fp16 backend implementation

Changes: https://git.openjdk.org/panama-vector/pull/204/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-vector&pr=204&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290204
  Stats: 536 lines in 16 files changed: 533 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/panama-vector/pull/204.diff
  Fetch: git fetch https://git.openjdk.org/panama-vector pull/204/head:pull/204

PR: https://git.openjdk.org/panama-vector/pull/204