gc: fix wrong arguments to error message for switches.
gc: fix wrong arguments to error message for switches.

Fixes issue 2502.

R=golang-dev, rsc
CC=golang-dev, remy
http://codereview.appspot.com/5472062

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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