Support better cross compilation, as well as other compilers (GCC)

[madsmtm]

Notably, GCC's libobjc runtime is very old and doesn't have objc_autorelease and similar, but maybe we could still use the limited Objective-C support is has for the few cases where we need it (e.g. the exception catching support)?

Would be especially useful for cross compilation scenarios, e.g. compiling for macOS on a Linux machine (though in that case you're likely to need clang anyhow, so maybe it's not so interesting? Will need some research!)

See also SSheldon/rust-objc-exception#12.

[madsmtm]

I don't think there's much for us to do here any more: In #648 I removed the build script helpers that objc-sys once had, and in d4b34b7 I made objc2-exception-helper able to compile without system headers. So the objc2 project itself is as cross-compilable as possible, if there's other issues it's with cc-rs or the Rust project in general.

[madsmtm]

I've also documented a bit about the cross-compilation process in 8ef2b23.