RFR: JDK-8304423: Refactor FdLibm.java

[Joe Darcy]

Minor refactoring of FdLibm.java; will consider future refactorings with more substantive code changes in the future.

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Commit messages:
 - JDK-8304423: Refactor Fdlibm.java

Changes: https://git.openjdk.org/jdk/pull/13604/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13604&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304423
  Stats: 85 lines in 1 file changed: 10 ins; 3 del; 72 mod
  Patch: https://git.openjdk.org/jdk/pull/13604.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13604/head:pull/13604

PR: https://git.openjdk.org/jdk/pull/13604