DERBY-6447: Use StrictMath for more functions in SYSFUN

Make LOG10, COSH, SINH and TANH use the corresponding methods in
java.lang.StrictMath instead of custom implementations.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1556810 13f79535-47bb-0310-9956-ffa450edef68