cmd/internal/ld, cmd/7l: external linking support for linux/arm64

Based on Michael Hudson-Doyle's patch:
b735215ee4

Change-Id: I309e3df7608b9eef9339196fdc50dedf5f9439f0
Reviewed-on: https://go-review.googlesource.com/8437
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/internal/ld/lib.go

@@ -711,6 +711,9 @@ func hostlink() {
 
 	case '5':
 		argv = append(argv, "-marm")
+
+	case '7':
+		// nothing needed
 	}
 
 	if Debug['s'] == 0 && debug_s == 0 {