configure: purge MMX/MMXEXT

With !424 (merged) merged we no longer have any MMX/MMXEXT to build, so we can get rid of the configure bits now.

Being able to remove the bits from src/misc/cpu.c is held up by !419 (merged), !420 (merged) and !421 (merged). I was going to wait to include it with this but I've got some other improvements to the cpu code to submit anyway, so I'll deal with it separately I guess.

I've bundled a commit to enable AVX for Windows since it touches a line already modified here and thus avoids a clash if I'd put that with !454.