cmd/go: disable verifyAsm for arm64

ARM64 doesn't have the old assembler.

Change-Id: I9253271029440e2b7f2813d3e98a7d2e7a65bfbc
Reviewed-on: https://go-review.googlesource.com/7045Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/build.go

@@ -1705,7 +1705,7 @@ func (gcToolchain) asm(b *builder, p *Package, obj, ofile, sfile string) error {
 	if err := b.run(p.Dir, p.ImportPath, nil, args...); err != nil {
 		return err
 	}
-	if verifyAsm {
+	if verifyAsm && goarch != "arm64" {
 		if err := toolVerify(b, p, "old"+archChar+"a", ofile, args); err != nil {
 			return err
 		}