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.