gas: Change to "swym 0" as canonical nop insn for MMIX

While "set $0, $0" works, that's not the documented instruction to do
nothing for MMIX.  However, I'm not changing it for "nop_type 5" as
seen in gas.exp and org-1.s, because "set $0, $0" seems like it could
be re-used there, for some future ISA.

gas/
	* config/tc-mmix.h (md_single_noop_insn): Change to "swym 0".