hack: define __EMSCRIPTEN__ when compiling to wasm

Too lazy to backport the Emscripten target to this version of LLVM

Author: zhuowei

lib/Basic/Targets/WebAssembly.cpp

@@ -60,6 +60,8 @@ void WebAssemblyTargetInfo::fillValidCPUList(
 void WebAssemblyTargetInfo::getTargetDefines(const LangOptions &Opts,
                                              MacroBuilder &Builder) const {
   defineCPUMacros(Builder, "wasm", /*Tuning=*/false);
+  // HACK: too lazy to backport the -emscripten target here
+  Builder.defineMacro("__EMSCRIPTEN__");
   if (SIMDLevel >= SIMD128)
     Builder.defineMacro("__wasm_simd128__");
   if (SIMDLevel >= UnimplementedSIMD128)