RFR: JDK-8283124: Add constant for tau to Math and StrictMath [v3]

[Joe Darcy]

> Add a constant for tau, 2*pi, to Math and StrictMath. Since 2*pi is a very common value in mathematical formulas, it is helpful to give it a distinct constant.
> 
> Please also review the CSR https://bugs.openjdk.java.net/browse/JDK-8283136

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 three additional commits since the last revision:

 - Merge branch 'master' into JDK-8283124
 - Set value with 2.0 * PI.
 - JDK-8283124: Add constant for tau to Math and StrictMath

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7813/files
  - new: https://git.openjdk.java.net/jdk/pull/7813/files/0fae656a..0cd1f61e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7813&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7813&range=01-02

  Stats: 1563 lines in 66 files changed: 1059 ins; 268 del; 236 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7813.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7813/head:pull/7813

PR: https://git.openjdk.java.net/jdk/pull/7813