Remove alignment from struct XSAVE altogether

I reduced the alignment of this struct in r342029 to avoid compiler
warnings about under-aligned allocations, but it turns out that this
still causes problems with some compilers (see r353778). As I hinted in
r342029, I don't believe any special aligment is necessary here (the
only reason for that would be if we used some aligned SSE instructions to
access this buffer, but I don't see any reason why we should do that),
so here I go all the way, and remove the alignment requirements (except
the ones naturally imposed by basic types) altogether.

llvm-svn: 354125