cmd/compile/internal/gc: fix go.y to match y.go

In https://go-review.googlesource.com/#/c/16554/ y.go was modified
manually, but go.y (which is the source for y.go) was not changed.

Change-Id: I1273801bfd1ac65b875f4465033b0d062abff0b7
Reviewed-on: https://go-review.googlesource.com/16745


Reviewed-by: Austin Clements <austin@google.com>