cmd/compile: use empty package name for runtime/{race,msan}

These fake imports are just so we can emit build dependencies for the
linker, so the package name isn't really necessary. Also, the package
import logic assumes that if we have the name for a package, then
we've already read some package data for it.

Using the empty string allows the importers to correctly populate it
the first time these packages are seen in package export data.

Passes toolstash-check.

Change-Id: I047bde297600e9dc07478fccc3f57ccc75ce8ae4
Reviewed-on: https://go-review.googlesource.com/107619
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/main.go

@@ -306,10 +306,10 @@ func Main(archInit func(*Arch)) {
 		flag_msan = false
 	}
 	if flag_race {
-		racepkg = types.NewPkg("runtime/race", "race")
+		racepkg = types.NewPkg("runtime/race", "")
 	}
 	if flag_msan {
-		msanpkg = types.NewPkg("runtime/msan", "msan")
+		msanpkg = types.NewPkg("runtime/msan", "")
 	}
 	if flag_race || flag_msan {
 		instrumenting = true