cmd/gc: fix comment for caninl
cmd/gc: fix comment for caninl

Was describing an old implementation.

R=ken2
CC=golang-dev
http://codereview.appspot.com/6553066

pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,dsriram/golang,gnanderson/go,gnanderson/go,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,gnanderson/go,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,dsriram/golang,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go