[tilegx] Keep Clang/LLVM va_list the same type with GCC.

  boo.c:
    void boo(va_list ap)
    {
      int a = va_arg(ap, int);
      printf("%d\n", a);
    }

  main.c:
    void boo(va_list ap);

    void foo(int arg1, ...)
    {
      va_list ap;
      va_start(ap, arg1);
      boo(ap);
      va_end(ap);
    }

   clang passed "ap" to function "boo" as pointer to structure, while
gcc expect it to be structure.

   we should keep clang and gcc the same ABI on this, otherwise they
cann't call each other when va_list arg involved in.