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

lib/Target/PNaCl/MCTargetDesc/PNaClMCTargetDesc.h

+//===-- PNaClMCTargetDesc.h - PNaCl Target Descriptions -------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file provides PNaCl specific target descriptions.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef PNACLMCTARGETDESC_H
+#define PNACLMCTARGETDESC_H
+
+namespace llvm {
+class MCSubtargetInfo;
+class Target;
+class StringRef;
+
+extern Target ThePNaClTarget;
+extern "C" void LLVMInitializePNaClTargetMC();
+} // End llvm namespace
+
+#endif