cmd/compile: mark ARM's CALLudiv not safepoint

ARM's udiv function is nosplit and it shouldn't be preemptied
(passing args in registers). It is in some sense like DUFFCOPY,
which we don't mark as safepoint.

Change-Id: I49f7c4e69e787ac364d0b0def0661e79a0ea9e69
Reviewed-on: https://go-review.googlesource.com/41370
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/arm/ssa.go

@@ -625,8 +625,10 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
 	case ssa.OpARMCALLstatic, ssa.OpARMCALLclosure, ssa.OpARMCALLinter:
 		s.Call(v)
 	case ssa.OpARMCALLudiv:
-		v.Aux = gc.Udiv
-		s.Call(v)
+		p := s.Prog(obj.ACALL)
+		p.To.Type = obj.TYPE_MEM
+		p.To.Name = obj.NAME_EXTERN
+		p.To.Sym = gc.Udiv
 	case ssa.OpARMDUFFZERO:
 		p := s.Prog(obj.ADUFFZERO)
 		p.To.Type = obj.TYPE_MEM

src/cmd/compile/internal/gc/plive.go

@@ -502,7 +502,7 @@ func onebitlivepointermap(lv *Liveness, liveout bvec, vars []*Node, args bvec, l
 // Returns true for instructions that are safe points that must be annotated
 // with liveness information.
 func issafepoint(v *ssa.Value) bool {
-	return v.Op.IsCall() || v.Op == ssa.OpARMCALLudiv
+	return v.Op.IsCall()
 }
 
 // Initializes the sets for solving the live variables. Visits all the