fixing stdio and adding a crude help message

definitely makes sense to remove that. gcc will emit 64 bit code on 64bit archs anyway, and user of unusual setup that tries to create 64bit code on 32bit arch can add the FLAGS to ARCHFLAG.
however to make that works ARCHFLAG should be expanded here, i.e.
```gcc -o $(SDC) obj/sdc.o $(ARCHFLAG) $(LDFLAGS)```