runtime: change int32 to intgo in findnull and findnullw

runtime: change int32 to intgo in findnull and findnullw

Update #6046.
This CL just does findnull and findnullw. There are other functions
to fix but doing them a few at a time will help isolate any (unlikely)
breakages these changes bring up in architectures I can't test
myself.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/12520043