add __UEFI__ to windows predefines

Updating the PE COFF generating windows code path to predefine the new __UEFI__ macro.

Author: Prabhuk

clang/lib/Basic/Targets/OSTargets.cpp

@@ -259,6 +259,7 @@ static void addVisualCDefines(const LangOptions &Opts, MacroBuilder &Builder) {
 
 void addWindowsDefines(const llvm::Triple &Triple, const LangOptions &Opts,
                        MacroBuilder &Builder) {
+  Builder.defineMacro("__UEFI__");
   Builder.defineMacro("_WIN32");
   if (Triple.isArch64Bit())
     Builder.defineMacro("_WIN64");