cmd/internal/objfile: add s390x support

Change-Id: I39aa6569c9a6f327f7aaa01f8b4ace814fd5b766
Reviewed-on: https://go-review.googlesource.com/20943Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/objfile/disasm.go

@@ -245,4 +245,5 @@ var byteOrders = map[string]binary.ByteOrder{
 	"arm":     binary.LittleEndian,
 	"ppc64":   binary.BigEndian,
 	"ppc64le": binary.LittleEndian,
+	"s390x":   binary.BigEndian,
 }

src/cmd/internal/objfile/elf.go

@@ -99,6 +99,8 @@ func (f *elfFile) goarch() string {
 		return "arm"
 	case elf.EM_PPC64:
 		return "ppc64"
+	case elf.EM_S390:
+		return "s390x"
 	}
 	return ""
 }