RFR: JDK-8302028: Port fdlibm atan2 to Java

[Joe Darcy]

Working down the porting list, next stop, atan2.

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Commit messages:
 - JDK-8302028: Port fdlibm atan2 to Java

Changes: https://git.openjdk.org/jdk/pull/12608/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12608&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302028
  Stats: 588 lines in 5 files changed: 571 ins; 8 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/12608.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12608/head:pull/12608

PR: https://git.openjdk.org/jdk/pull/12608