Prefix most of the preprocessor macros with `M68K_`

[eschaton]

To ensure ease of interoperability, I've prefixed almost all of the preprocessor macros used by Musashi with M68K_ since that's the common prefix in the public API. The only ones I've left alone are TRUE and FALSE and NULL since those are so universal.

I've also wrapped the preprocessor macros with #ifdef and #endif so the contents of m68kconf.h can be selectively overridden, meaning those using Musashi don't need to incorporate and change it or write their own from scratch and replace it.