[lworld+fp16] RFR: 8330021: AArch64: Add backend support for FP16 add operation

[Bhavana Kilambi]

This commit [1] adds initial support for FP16 operations and adds backend support for FP16 add operation for X86. This task adds backend support for scalar and vector FP16 add operation on aarch64.

[1] https://github.com/openjdk/valhalla/commit/f03fb4e4ee4d59ed692d0c26ddce260511f544e7#diff-a799ce8da7f3062bb3699beb65aae504840c649942032e325c2a50f88a2869ad

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Commit messages:
 - 8330021: AArch64: Add backend support for FP16 add operation

Changes: https://git.openjdk.org/valhalla/pull/1096/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1096&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330021
  Stats: 968 lines in 19 files changed: 271 ins; 4 del; 693 mod
  Patch: https://git.openjdk.org/valhalla/pull/1096.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1096/head:pull/1096

PR: https://git.openjdk.org/valhalla/pull/1096