Remove FixedMul and FixedDiv2 asm implementations

Actually works this time.

As for why this should be done, effectively there is no reason to have hand-optimized versions of these functions anymore. Modern compilers are very good at optimizing code fragments such as this, and based on what I've seen gcc is already doing it with the existing asm. That is to say, it has outlived its purpose and now is just clutter.