RFR: 8290079: Reduce interaction with volatile in static initializer of BigInteger

[Сергей Цыпанов]

`BigInteger.powerCache` is volatile and should be assigned only once in static initializer.

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

Commit messages:
 - 8290079: Reduce interaction with volatile in static initializer of BigInteger

Changes: https://git.openjdk.org/jdk/pull/9451/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9451&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290079
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9451.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9451/head:pull/9451

PR: https://git.openjdk.org/jdk/pull/9451