RFR: 8253407: Move max_pow2 from test to src

[Stefan Karlsson]

Small refactoring to move test utility function over to src, so that it can be used in both places.

Patch summary:
- Moved the code
- Replaced duplicate code in round_power_of_2
- Adjusted to the surrounding powerOfTwo.hpp code and only accept integral types.
- Added small gtest. (On the fence whether it's worth it or not)

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Commit messages:
 - 8253407: Move max_pow2 from test to src

Changes: https://git.openjdk.java.net/jdk/pull/279/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=279&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253407
  Stats: 31 lines in 2 files changed: 17 ins; 6 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/279.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/279/head:pull/279

PR: https://git.openjdk.java.net/jdk/pull/279