78bbe57c88
This adds support for rust-lld for Apple *OS targets. This was tested against targets "aarch64-apple-ios" and "aarch64-apple-ios-sim". For targets "armv7-apple-ios" and "armv7s-apple-ios", it doesn't link because of "symbols.o" not being generated with the correct CPU subtype (changes in the "object" crate needs to be done to support it).
rustc_target contains some very low-level details that are
specific to different compilation targets and so forth.
For more information about how rustc works, see the rustc dev guide.