[M68k] Adopt the new VarLenCodeEmitterGen for arithmetic instructions

This patch refactors all the existing M68k arithmetic instructions
to use the new VarLenCodeEmitterGen infrastructure.

This patch is tested by the existing MC test cases.

Note that one of the codegen tests needed to be updated because the
ordering of two equivalent instructions were switched.

Differential Revision: https://reviews.llvm.org/D115234