Deprecate -msse2 and -msse flags

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.

Trac metadata

Trac field	Value
Version	7.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (CodeGen)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonmar
Operating system
Architecture