RFR: JDK-8302028: Port fdlibm atan2 to Java [v5]

[Joe Darcy]

> Working down the porting list, next stop, atan2.

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Respond to review feedback.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12608/files
  - new: https://git.openjdk.org/jdk/pull/12608/files/82790bbb..35c2bfef

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12608&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12608&range=03-04

  Stats: 32 lines in 2 files changed: 8 ins; 0 del; 24 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