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