C: add Keith Randall (Google CLA)

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7595043
cmd/8g: fix code generation of int64(0) == int64(0).

The code would violate the contract of cmp64.

Fixes issue 5002.

R=rsc, golang-dev
CC=golang-dev
https://codereview.appspot.com/7593043