This makes miri not diverge in behavior, it fixes running Rust linux-gnu
binaries on musl with gcompat, it fixes dlopen edge-cases that cranelift
somehow hits, etc.
This is very similar to what Apple's own headers encourage you to do (cast the function pointer before use instead of making new declarations).
Additionally, I'm documenting a few of the memory management rules we're following, ensuring that the `args` function doesn't leak memory (if you wrap it in an autorelease pool).
