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

[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

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Commit messages:
 - JDK-8283124: Add constant for tau to Math and StrictMath

Changes: https://git.openjdk.java.net/jdk/pull/7813/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7813&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283124
  Stats: 29 lines in 2 files changed: 26 ins; 0 del; 3 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