[Bug target/53383] Allow -mpreferred-stack-boundary=3 on x86-64

[hubicka at ucw dot cz]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53383

--- Comment #8 from Jan Hubicka <hubicka at ucw dot cz> 2012-05-19 19:14:18 UTC ---
__int128 do not require SSE and yet it is 128bit aligned....
I am not against allowing smaller alignments, we just need to document it
breaks ABI and it would be nice to explain how
(and probably warn on 128bit aligned varargs or force the dynamc alignment)