RFR: JDK-8302028: Port fdlibm atan2 to Java [v4]

[Joe Darcy]

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

Joe Darcy has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Use NaN regression test infrastructure.
 - Merge branch 'master' into JDK-8302028
 - Correct and test for issue found in code review.
 - Update regression test.
 - Merge branch 'master' into JDK-8302028
 - JDK-8302028: Port fdlibm atan2 to Java

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/12608/files
  - new: https://git.openjdk.org/jdk/pull/12608/files/2c524913..82790bbb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12608&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12608&range=02-03

  Stats: 6881 lines in 292 files changed: 3843 ins; 1365 del; 1673 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