cmd/6g has been doing this for a long time.

Arrays are still problematic on 5g because the addressing
for t[0] where local var t has type [3]uintptr takes the address of t.
That's issue 8125.

Fixes #8123.

LGTM=josharian
R=josharian, dave
CC=golang-codereviews
https://golang.org/cl/102890046