Fix MIPS64 boot

Return register in FieldAccessCallingConventionMIPS64 was A0,
but it should be V0.

With this change, the system server doesn't crash.

Change-Id: Id52f684658d235fd001d9784145f4ea5ed2938b6
diff --git a/compiler/optimizing/code_generator_mips64.h b/compiler/optimizing/code_generator_mips64.h
index 5e8f9e7..7799437 100644
--- a/compiler/optimizing/code_generator_mips64.h
+++ b/compiler/optimizing/code_generator_mips64.h
@@ -117,7 +117,7 @@
     return Location::RegisterLocation(A0);
   }
   Location GetReturnLocation(Primitive::Type type ATTRIBUTE_UNUSED) const OVERRIDE {
-    return Location::RegisterLocation(A0);
+    return Location::RegisterLocation(V0);
   }
   Location GetSetValueLocation(
       Primitive::Type type ATTRIBUTE_UNUSED, bool is_instance) const OVERRIDE {