Update x86.S

GCC now assumes by default that the stack is aligned to a 16-byte boundary in Linux x86, so let's do our part to honour that. :)
Otherwise native code that depends on the stack to be aligned to 16 bytes would seg fault (like SSE).
https://groups.google.com/forum/#!topic/avian/SyCl-Jfw2U8
Merge pull request #75 from frustaci/patch-1

Fix 16-byte boundary in Linux x86