[MINOR] [SQL] Support mutable expression unit test with codegen projection

This is actually contains 3 minor issues:
1) Enable the unit test(codegen) for mutable expressions (FormatNumber, Regexp_Replace/Regexp_Extract)
2) Use the `PlatformDependent.copyMemory` instead of the `System.arrayCopy`

Author: Cheng Hao <hao.cheng@intel.com>

Closes #7566 from chenghao-intel/codegen_ut and squashes the following commits:

24f43ea [Cheng Hao] enable codegen for mutable expression & UTF8String performance