Small change to how to define the alignment of an object on the stack

I have just removed the 's' modifier in commit r198287.

Any out of tree target that is using this feature needs a small
update. If target Foo has a value X for 's' in its datalayout, it
should now instead define a function

unsigned FooTargetLowering::getByValTypeAlignment(Type *Ty) const {
  return X/8;