sh: Make sure rte delay slots are nopped out on all parts.

Future SH parts do not support any instruction but a nop in the rte delay
slot, so make the change for all offending parts. SH-5 is excluded from
this, and already has its own set of restrictions with regards to rte
delay slot handling.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>