Don't distinguish between i586/i686

This was historically done as the contents of the `i686` module wouldn't
actually compile on i586 for various reasons. I believe I've tracked this down
to #300 where LLVM refuses to compile a function using the `x86_mmx` type
without actually enabling the `mmx` feature (sort of reasonably so!). This
commit will now compile in both the `i586` and `i686` modules of this crate into
the `i586-unknown-linux-gnu` target, and the relevant functions now also enable
the `mmx` feature if they're using the `__m64` type.

I believe this is uncovering a more widespread problem where the `__m64` isn't
usable outside the context of `mmx`-enabled functions. The i686 and x86_64
targets have this feature enabled by default which is why it's worked there, but
they're not enabled for the i586 target. We'll probably want to consider this
when stabilizing!

Author: alexcrichton