Use compiler __SSE2__ definitions and C++11 alignas

Summary:
Since C++11 alignas is a keyword, so we don't need to create our own.
All relevant compilers (gcc, clang, icc) define __SSE2__, so just use that.

There seem to be frequent crashes in the quad version of this code, but
this change should not make any difference from what I can tell.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23120