ARM: VIXL32: Implement the functionality needed by intrinsics.

This patch passes:
* 004-UnsafeTest

Test: ART_USE_VIXL_ARM_BACKEND=true m test-art-host
Test: ART_USE_VIXL_ARM_BACKEND=true m test-art-target
Change-Id: I7a3cd410411ef3a520d419734a835090097b2aee
diff --git a/compiler/utils/arm/assembler_arm_vixl.h b/compiler/utils/arm/assembler_arm_vixl.h
index 68fd32e..e020628 100644
--- a/compiler/utils/arm/assembler_arm_vixl.h
+++ b/compiler/utils/arm/assembler_arm_vixl.h
@@ -72,6 +72,8 @@
   void PoisonHeapReference(vixl32::Register reg);
   // Unpoison a heap reference contained in `reg`.
   void UnpoisonHeapReference(vixl32::Register reg);
+  // Poison a heap reference contained in `reg` if heap poisoning is enabled.
+  void MaybePoisonHeapReference(vixl32::Register reg);
   // Unpoison a heap reference contained in `reg` if heap poisoning is enabled.
   void MaybeUnpoisonHeapReference(vixl32::Register reg);