[M68k] Make M68k TargetMachine use getter function

[M68k] Make M68k TargetMachine use getter function

This makes M68k match other platforms in this regard.

This was done as part of the AsmParser/Disassembler work since the entry
functions of those modules usually reference `getTheXXXTarget()`.

Differential Revision: https://reviews.llvm.org/D98517