[benchmark] Replace references to M680x0 with M68k

The former was the old unusual name of the out-of-tree backend but it
was renamed to M68k during the code review process to conform with how
almost everything refers to the Motorola 68000 family of processors.
Thus, update the comments to avoid confusion when the backend lands.

GitOrigin-RevId: 5d6e0e474e8636077ad00b9b4f18bcd1e30cb482
1 file changed