Add a va_arg intrinsic

[mahkoh]

fn va_arg<T>(va_list: *mut u8) -> T;

translates to

va_arg i8* va_list, T

This is useful for C interop. va_arg can theoretically be implemented in user code but that's a lot of work.

[mahkoh]

It's not possible to implement this in user code without some kind of reflection/type information.

[fstirlitz]

I believe #2137 addresses this.

[Centril]

Closing in favor of rust-lang/rust#44930.