cmd/internal/objfile, debug/macho: support disassembling arm64 Mach-O objects

Fixes #25423

Change-Id: I6bed0726b8f4c7d607a3df271b2ab1006e96fa75
Reviewed-on: https://go-review.googlesource.com/113356
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/objfile/macho.go

@@ -113,6 +113,8 @@ func (f *machoFile) goarch() string {
 		return "amd64"
 	case macho.CpuArm:
 		return "arm"
+	case macho.CpuArm64:
+		return "arm64"
 	case macho.CpuPpc64:
 		return "ppc64"
 	}

src/debug/macho/macho.go

@@ -60,6 +60,7 @@ const (
 	Cpu386   Cpu = 7
 	CpuAmd64 Cpu = Cpu386 | cpuArch64
 	CpuArm   Cpu = 12
+	CpuArm64 Cpu = CpuArm | cpuArch64
 	CpuPpc   Cpu = 18
 	CpuPpc64 Cpu = CpuPpc | cpuArch64
 )
@@ -68,6 +69,7 @@ var cpuStrings = []intName{
 	{uint32(Cpu386), "Cpu386"},
 	{uint32(CpuAmd64), "CpuAmd64"},
 	{uint32(CpuArm), "CpuArm"},
+	{uint32(CpuArm64), "CpuArm64"},
 	{uint32(CpuPpc), "CpuPpc"},
 	{uint32(CpuPpc64), "CpuPpc64"},
 }