cmd/vet: keep processing files after warning

This lets go vet produce useful results for syscall on darwin/amd64,
otherwise it gets caught on asm.s.

Change-Id: I3daa49cf61eeada52eb9d4e94e40c4d21d1d5b2d
Reviewed-on: https://go-review.googlesource.com/2741
Reviewed-by: Rob Pike <r@golang.org>

cmd/vet/asmdecl.go

@@ -110,6 +110,7 @@ func asmCheck(pkg *Package) {
 		}
 	}
 
+Files:
 	for _, f := range pkg.files {
 		if !strings.HasSuffix(f.name, ".s") {
 			continue
@@ -173,7 +174,7 @@ func asmCheck(pkg *Package) {
 				flushRet()
 				if arch == "" {
 					f.Warnf(token.NoPos, "%s: cannot determine architecture for assembly file", f.name)
-					return
+					continue Files
 				}
 				fnName = m[1]
 				fn = knownFunc[m[1]][arch]