[vectorIntrinsics+fp16] RFR: 8305563: [vectorapi]: Initial aarch64 backend implementation for FP16 operations

[Bhavana Kilambi]

This patch adds aarch64 (Neon and SVE) backend implementation for the following FP16 operations for which C2 support was added in the commit https://github.com/openjdk/panama-vector/commit/7460d9327aac7d1d2ba6aed4e7137a417dbf6a97 -

Unary operations : AbsVHF, NegVHF
Binary operations : AddVHF, SubVHF, MulVHF, DivVHF
Ternary operations : AddReductionVHF, FmaVHF
Conversion operations : VectorCastHF2D, VectorCastD2HF

Some of these operations on Neon machines, require features "fphp" and "asimdhp" to be available. Feature detection for these features have also been added.

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Commit messages:
 - 8305563: [vectorapi]: Initial aarch64 backend implementation for FP16 operations

Changes: https://git.openjdk.org/panama-vector/pull/217/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-vector&pr=217&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305563
  Stats: 1170 lines in 11 files changed: 388 ins; 0 del; 782 mod
  Patch: https://git.openjdk.org/panama-vector/pull/217.diff
  Fetch: git fetch https://git.openjdk.org/panama-vector.git pull/217/head:pull/217

PR: https://git.openjdk.org/panama-vector/pull/217