cmd/gc: fix ARM build

CL 2520 omitted to set the type for an OCONVNOP node.
Typechecking obviously cannot do it for us.

5g inserts float64 <--> [u]int64 conversions at walk time.
The missing type caused it to crash.

Change-Id: Idce381f219bfef2e3a3be38d3ba3c258b71310ae
Reviewed-on: https://go-review.googlesource.com/2640


Reviewed-by: Keith Randall <khr@golang.org>