Change BKPT and UDF encodings on ARM.

src/trusted/validator_arm/validator.cc

Index: src/trusted/validator_arm/validator.cc
diff --git a/src/trusted/validator_arm/validator.cc b/src/trusted/validator_arm/validator.cc
index 0742be242a0485525b7efdb034cbb0c1a9bfc7b6..7f99520b9b9b006646b6063bce200bf1c443deab 100644
--- a/src/trusted/validator_arm/validator.cc
+++ b/src/trusted/validator_arm/validator.cc
@@ -675,10 +675,10 @@ bool SfiValidator::validate_fallthrough(const CodeSegment& segment,
   bool complete_success = true;
 
   nacl_arm_dec::Forbidden initial_decoder;
-  // Initialize the previous instruction to a scary BKPT, so patterns all fail.
+  // Initialize the previous instruction so it always fails validation.
   DecodedInstruction pred(
       0,  // Virtual address 0, which will be in a different bundle;
-      Instruction(nacl_arm_dec::kLiteralPoolHeadInstruction),
+      Instruction(nacl_arm_dec::kFailValidation),
       initial_decoder);  // and ensure that it decodes as Forbidden.
 
   for (uint32_t va = segment.begin_addr(); va < segment.end_addr(); va += 4) {