arm64/arm64asm: use llvm-objdump for objdump tests on openbsd

The GNU objdump that ships with openbsd/arm64 is old and not fully
functional. Use llvm-objdump instead.

Change-Id: I500e93b01445266245c4f66e936faf380c5fd337
Reviewed-on: https://go-review.googlesource.com/c/arch/+/805181
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Carlos Amedee <carlos@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

arm64/arm64asm/objdumpext_test.go

diff --git a/arm64/arm64asm/objdumpext_test.go b/arm64/arm64asm/objdumpext_test.go
index 3bf4fa8..8fc71c7 100644
--- a/arm64/arm64asm/objdumpext_test.go
+++ b/arm64/arm64asm/objdumpext_test.go
@@ -15,12 +15,19 @@
 	"log"
 	"os"
 	"os/exec"
+	"runtime"
 	"strconv"
 	"strings"
 	"testing"
 )
 
-const objdumpPath = "/usr/bin/objdump"
+var objdumpPath = "/usr/bin/objdump"
+
+func init() {
+	if runtime.GOOS == "openbsd" {
+		objdumpPath = "/usr/bin/llvm-objdump"
+	}
+}
 
 func testObjdumpARM64(t *testing.T, generate func(func([]byte))) {
 	testObjdumpArch(t, generate, ModeARM64)