RFR: 8345146: [PPC64] Make intrinsic conversions between bit representations of half precision values and floats

[Martin Doerr]

PPC64 implementation of [JDK-8289552](https://bugs.openjdk.org/browse/JDK-8289552). I've implemented some more instructions which may be useful in the future.

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Commit messages:
 - 8345146: [PPC64] Make intrinsic conversions between bit representations of half precision values and floats

Changes: https://git.openjdk.org/jdk/pull/22433/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22433&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345146
  Stats: 180 lines in 15 files changed: 177 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/22433.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22433/head:pull/22433

PR: https://git.openjdk.org/jdk/pull/22433