lto/addAsmGlobalSymbols: fail fracefully when the target does not define AsmParser.

lib/Target/PNaCl/TargetInfo/PNaClTargetInfo.cpp

+//===-- PNaClTargetInfo.cpp - PNaCl Target Implementation -------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "PNaClTargetMachine.h"
+#include "llvm/Module.h"
+#include "llvm/Support/TargetRegistry.h"
+using namespace llvm;
+
+Target llvm::ThePNaClTarget;
+
+extern "C" void LLVMInitializePNaClTargetInfo() {
+  RegisterTarget<Triple::le32> X(ThePNaClTarget, "pnacl", "PNaCl backend");
+}