Common: Introduce the new Gekko disassembler to Common.

This moves the Gekko disassembler to Common where it should be. Having it in the Bochs disassembly Externals is incorrect.

Unlike the PowerPC disassembler prior however, this one is updated to have an API that is more fitting for C++. e.g. Not needing to specify a string buffer and size. It does all of this under the hood.

This modifies all the DebuggingInterfaces as necessary to handle this.
Merge pull request #647 from lioncash/disasm

Common: Introduce the new Gekko disassembler to Common.