[committed] Fix minor bug on mn103 port

Richard Sandiford debugged a failure on the mn103 port with his late-combine
patches down to the subdi3 pattern not specifying the isa on alternatives which
required newer variants of the chip family.

This patch adds the missing isa attribute and the port now works with his
late-combine patch.  I'm pushing this to the trunk on his behalf.

gcc/
	* config/mn10300/mn10300.md (subdi3_degenerate): Add isa attribute.