switch -msse2 to be on by default
I propose msse2 to be on by default. I guess the default was chosen way back, when Pentium III support was still relevant.
Nowadays we don't really win on the CPU support, because e.g. https://github.com/tibbe/hashable/blob/master/hashable.cabal is built by default with sse2 on the injected C code level. And hashable has a lot of reverse depends, therefore on an end user system (RedHat or Debian) the user is most probably unlucky with a Pentium III CPU anyways.
Flipping this default would also fix the excess precision problem for most users of GHC on the i686 platform.
GHC should provide a -mno-sse2 flag for the cases when this needs to be disabled.