cmd/link/internal/s390x: fix s390x build

Fix breakage from CL 173437

Change-Id: If218ffaa1259fbdee641143ffbe4b38030c373b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/174278Reviewed-by: Michael Munday <mike.munday@ibm.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/link/internal/s390x/asm.go

@@ -69,6 +69,7 @@ func gentext(ctxt *ld.Link) {
 	initfunc.AddUint8(0xc0)
 	initfunc.AddUint8(0x20)
 	lmd := initfunc.AddRel()
+	lmd.InitExt()
 	lmd.Off = int32(initfunc.Size)
 	lmd.Siz = 4
 	lmd.Sym = ctxt.Moduledata
@@ -81,6 +82,7 @@ func gentext(ctxt *ld.Link) {
 	initfunc.AddUint8(0xc0)
 	initfunc.AddUint8(0xf4)
 	rel := initfunc.AddRel()
+	rel.InitExt()
 	rel.Off = int32(initfunc.Size)
 	rel.Siz = 4
 	rel.Sym = ctxt.Syms.Lookup("runtime.addmoduledata", 0)