Remove Emscripten support

lib/Target/JSBackend/JSTargetMachine.h

-//===-- JSTargetMachine.h - TargetMachine for the JS Backend ----*- C++ -*-===//
-//
-//                     The LLVM Compiler Infrastructure
-//
-// This file is distributed under the University of Illinois Open Source
-// License. See LICENSE.TXT for details.
-//
-//===---------------------------------------------------------------------===//
-//
-// This file declares the TargetMachine that is used by the JS/asm.js/
-// emscripten backend.
-//
-//===---------------------------------------------------------------------===//
-
-#ifndef JSTARGETMACHINE_H
-#define JSTARGETMACHINE_H
-
-#include "JS.h"
-#include "llvm/Target/TargetMachine.h"
-
-namespace llvm {
-
-class formatted_raw_ostream;
-
-class JSTargetMachine : public TargetMachine {
-public:
-  JSTargetMachine(const Target &T, StringRef Triple,
-                  StringRef CPU, StringRef FS, const TargetOptions &Options,
-                  Reloc::Model RM, CodeModel::Model CM,
-                  CodeGenOpt::Level OL);
-
-  bool addPassesToEmitFile(PassManagerBase &PM, raw_pwrite_stream &Out,
-                           CodeGenFileType FileType, bool DisableVerify,
-                           AnalysisID StartAfter,
-                           AnalysisID StopAfter) override;
-
-  TargetIRAnalysis getTargetIRAnalysis() override;
-};
-
-} // End llvm namespace
-
-#endif