Fixed issue 11155

This fixes [issue 11155](http://forum.dlang.org/thread/bug-11155-3@http.d.puremagic.com/issues/).

I aligned the Fast section in prolog(). Some comments in the source code indicate that aligning this section to more than REGSIZE breaks contract implementation. I don't believe that to be the case (see the comment I have added to cgcod.c), but still, this pull request should definitely be reviewed by someone who knows the backend well.