RFR: JDK-8308049: Refactor nested class declarations in FdLibm.java

[Joe Darcy]

Make the nested classes inside FdLibm.java consistently package-access, static and final.

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Commit messages:
 - JDK-8308049: Refactor nested class declarations in FdLibm.java

Changes: https://git.openjdk.org/jdk/pull/13972/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13972&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308049
  Stats: 17 lines in 1 file changed: 0 ins; 0 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/13972.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13972/head:pull/13972

PR: https://git.openjdk.org/jdk/pull/13972