RFR: 8346914: UB issue in scalbnA

[Kim Barrett]

Please review this change that replaces uses of our scalbnA function with
using the standard scalbn function. Removed scalbnA, and also copysignA.

For more details, see first comment and JBS.

Testing: mach5 tier1-6. Though from discussion in
https://github.com/openjdk/jdk/pull/25656, it's hard to get to our uses of
scalbn/scalbnA.

Before removing it, I tested scalbnA via a gtest that is attached to the JBS
issue.

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

Commit messages:
 - update copyrights
 - remove unused twom54 constant
 - scalbnA => scalbn, remove copysignA

Changes: https://git.openjdk.org/jdk/pull/25917/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25917&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346914
  Stats: 67 lines in 4 files changed: 0 ins; 55 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/25917.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25917/head:pull/25917

PR: https://git.openjdk.org/jdk/pull/25917