armemu: Implement USAD8 and USADA8

Also thanks go out to @unknownbrackets for helping me get up to speed with some ARM stuff, otherwise I wouldn't have been able to do this.
Yeah, simple uses of lambdas tend to be well inlined.
@unknownbrackets The question was more of a rethorical one. I consider it bad to add tricky bit-twiddling tricks without any explanation of how they work. However, thanks for the explanation.