RFR: JDK-8283124: Add constant for tau to Math and StrictMath [v2]

[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 incrementally with one additional commit since the last revision:

  Set value with 2.0 * PI.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7813/files
  - new: https://git.openjdk.java.net/jdk/pull/7813/files/c5931750..0fae656a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7813&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7813&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 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