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

lib/Target/PNaCl/MCTargetDesc/PNaClMCAsmInfo.cpp

+//===-- PNaClMCAsmInfo.cpp - PNaCl asm properties ---------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file contains the declarations of the PNaClMCAsmInfo properties.
+//
+//===----------------------------------------------------------------------===//
+
+#include "PNaClMCAsmInfo.h"
+using namespace llvm;
+
+PNaClMCAsmInfo::PNaClMCAsmInfo(const Target &T, StringRef TT) {
+  // PNaCl does not support assembly. It's a stub to make LTO works.
+}