Even if requested, there is no .go file for unsafe - it comes from the
compiler - so referencing its cover variables will break the compilation
in a command like
	go test -coverpkg=all fmt

Fixes #10408.

Change-Id: If92658ef6c29dc020f66ba30b02eaa796f7205e0
Reviewed-on: https://go-review.googlesource.com/8891Reviewed-by: Russ Cox <rsc@golang.org>