vet: add a progress message to asmdecl.go

Analogous to the one for .go files, it's for .s only and is protected
by the verbose flag.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/8030046

src/cmd/vet/asmdecl.go

@@ -104,6 +104,7 @@ func asmCheck(pkg *Package) {
 		if !strings.HasSuffix(f.name, ".s") {
 			continue
 		}
+		Println("Checking file", f.name)
 
 		// Determine architecture from file name if possible.
 		var arch string