Add missing definition of CodeGeneratorARMVIXL::GenerateReadBarrierSlow.

This fixes build breakages on the read barrier table lookup
configuration.  This is just a dummy definition, as ART's
AArch32 VIXL back end is work in progress.

Test: ART_USE_READ_BARRIER=true ART_READ_BARRIER_TYPE=TABLELOOKUP make
Change-Id: Ifef39148fb7d9d8a5ad309ec0a2edc8a97d72f3d
diff --git a/compiler/optimizing/code_generator_arm_vixl.cc b/compiler/optimizing/code_generator_arm_vixl.cc
index 891cf77..46a68c1 100644
--- a/compiler/optimizing/code_generator_arm_vixl.cc
+++ b/compiler/optimizing/code_generator_arm_vixl.cc
@@ -5062,6 +5062,15 @@
   }
 }
 
+void CodeGeneratorARMVIXL::GenerateReadBarrierSlow(HInstruction* instruction ATTRIBUTE_UNUSED,
+                                                   Location out ATTRIBUTE_UNUSED,
+                                                   Location ref ATTRIBUTE_UNUSED,
+                                                   Location obj ATTRIBUTE_UNUSED,
+                                                   uint32_t offset ATTRIBUTE_UNUSED,
+                                                   Location index ATTRIBUTE_UNUSED) {
+  TODO_VIXL32(FATAL);
+}
+
 void CodeGeneratorARMVIXL::MaybeGenerateReadBarrierSlow(HInstruction* instruction ATTRIBUTE_UNUSED,
                                                         Location out,
                                                         Location ref ATTRIBUTE_UNUSED,