Restore MMX support

[fw-immunant]

We're going to drop MMX support due to the straightforward translation becoming impossible as upstream has also dropped support for these types (e.g. __m64) and intrinsics.

This isn't likely to be disruptive to most users, even of SIMD, because MMX is quite old and generally superseded by newer instruction set extensions. In addition, most C programs with explicit SIMD have it hidden behind architecture-specific #ifdefs, and those programs can still be translated by configuring away the MMX.

If there's demand from users that really need to translate C programs containing explicit MMX intrinsics and types, we can investigate other possible ways to expose these. In the meantime, this issue can serve to gauge interest and document the current situation.