[PATCH] rx: allow cmpstrnsi len to be zero

[PATCH] rx: allow cmpstrnsi len to be zero

The SCMPU instruction doesn't change the C and Z flags when the
incoming length is zero, which means the insn will produce a
value based upon the existing flag values.

As a quick kludge, adjust these flags to ensure a zero result in this
case.

gcc/
	* config/rx/rx.md (rx_cmpstrn): Correctly handle len=0 case.