RFR: 8375223: Add proper toString output to bfloat16 implementation

[Joe Darcy]

Add toString() implementation that follows "shortest string" conventions.

Follow-up testing to be done under JDK-8375224: Add tests for Bfloat16.toString().

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Commit messages:
 - 8375223: Add proper toString output to bfloat16 implementation

Changes: https://git.openjdk.org/valhalla/pull/1900/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1900&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375223
  Stats: 159 lines in 1 file changed: 75 ins; 23 del; 61 mod
  Patch: https://git.openjdk.org/valhalla/pull/1900.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1900/head:pull/1900

PR: https://git.openjdk.org/valhalla/pull/1900