do not generate fusion.md, update contrib/gcc_update

In a previous fusion-combine patch for rs6000, Segher had asked me to
comment out the automatic regeneration of fusion.md. And more recently
Edelsohn pointed out that gcc_update needed to fix the timestamp of
fusion.md so it didn't get unnecessarily regenerated.

contrib/ChangeLog:

	* gcc_update (files_and_dependencies): Add dependency for
	gcc/config/rs6000/fusion.md on gcc/config/rs6000/genfusion.md.

gcc/ChangeLog:

	* config/rs6000/t-rs6000: Comment out auto generation of
	fusion.md for now.