Remove the "stack alignment" data layout spec

[robdockins]

It seems that this aspect of the data layout string was removed starting with LLVM 3.5.0. As best I can tell, we only support LLVM back to the 3.5 series, so we can and probably should remove this layout spec.

To be clear, I'm refering to the lower case s specification (which behaves similar to the i, f and a specifications), not the upper-case S specification, which means something quite different, and is still supported.