arm: Move unrolled array initialization to Assembler.

runtime/vm/assembler_arm.h

Index: runtime/vm/assembler_arm.h
===================================================================
--- runtime/vm/assembler_arm.h	(revision 41908)
+++ runtime/vm/assembler_arm.h	(working copy)
@@ -672,11 +672,18 @@
   // range [begin, end), assumed to be uninitialized fields in object (tagged).
   // The stores must not need a generational store barrier (e.g., smi/null),
   // and (value_even, value_odd) must be a valid register pair.
+  // Destroys register 'begin'.
   void InitializeFieldsNoBarrier(Register object,
                                  Register begin,
                                  Register end,
                                  Register value_even,
                                  Register value_odd);
+  // Like above, for the range [begin, begin+count*kWordSize), unrolled.
+  void InitializeFieldsNoBarrierUnrolled(Register object,
+                                         Register begin,
+                                         intptr_t count,
+                                         Register value_even,
+                                         Register value_odd);
 
   void LoadClassId(Register result, Register object, Condition cond = AL);
   void LoadClassById(Register result, Register class_id);