RFR: 8356104: Add bfloat16 implementation for testing purposes

[Joe Darcy]

Add a stand-alone Bfloat16 implementation for testing purposes.

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Commit messages:
 - JDK-8356104: Add bfloat16 implementation for testing purposes

Changes: https://git.openjdk.org/valhalla/pull/1890/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1890&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356104
  Stats: 4029 lines in 4 files changed: 4029 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/1890.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1890/head:pull/1890

PR: https://git.openjdk.org/valhalla/pull/1890