RFR: 8285477: Add a PRECISION public static field to j.l.Float and j.l.Double

[Raffaello Giulietti]

Add useful constants specified in IEEE 754.

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Commit messages:
 - 8285477: Add a PRECISION public static field to j.l.Float and j.l.Double

Changes: https://git.openjdk.java.net/jdk/pull/8362/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8362&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285477
  Stats: 50 lines in 2 files changed: 32 ins; 14 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8362.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8362/head:pull/8362

PR: https://git.openjdk.java.net/jdk/pull/8362