cmd/cgo: change GoStringN and GoBytes from intgo to int32
cmd/cgo: change GoStringN and GoBytes from intgo to int32

Fixes build.

R=golang-dev
CC=golang-dev
https://codereview.appspot.com/9667047

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang