Update V8 to r4851 as required by WebKit r61121

Change-Id: Ib01b7c8e38d5b82b254192fc06365aa5b85780c5
diff --git a/src/arm/constants-arm.h b/src/arm/constants-arm.h
index 57c5c1c..e36f595 100644
--- a/src/arm/constants-arm.h
+++ b/src/arm/constants-arm.h
@@ -66,10 +66,15 @@
 # define CAN_USE_THUMB_INSTRUCTIONS 1
 #endif
 
-// Simulator should support ARM5 instructions.
+// Simulator should support ARM5 instructions and unaligned access by default.
 #if !defined(__arm__)
 # define CAN_USE_ARMV5_INSTRUCTIONS 1
 # define CAN_USE_THUMB_INSTRUCTIONS 1
+
+# ifndef CAN_USE_UNALIGNED_ACCESSES
+#  define CAN_USE_UNALIGNED_ACCESSES 1
+# endif
+
 #endif
 
 #if CAN_USE_UNALIGNED_ACCESSES