Tentative fix for build breakage with read barriers.

Change-Id: I490821357525bdeb19c330f350635bd645f8ab53
diff --git a/runtime/arch/arm/quick_entrypoints_arm.S b/runtime/arch/arm/quick_entrypoints_arm.S
index bd1b33f..82ec0b7 100644
--- a/runtime/arch/arm/quick_entrypoints_arm.S
+++ b/runtime/arch/arm/quick_entrypoints_arm.S
@@ -1125,7 +1125,7 @@
     // r0: type_idx/return value, r1: ArtMethod*, r9: Thread::Current
     // r2, r3, r12: free.
 #if defined(USE_READ_BARRIER)
-    mvn    x0, #0                                             // Read barrier not supported here.
+    mvn    r0, #0                                             // Read barrier not supported here.
     bx     lr                                                 // Return -1.
 #endif
     ldr    r2, [r1, #ART_METHOD_DEX_CACHE_TYPES_OFFSET_32]    // Load dex cache resolved types array