Right now if you configure multiple hosts rustbuild will only build
documentation for the build triple, but we've got all the support necessary to
build documentation for different architectures as well. This commit
reinterprets the `target` field of doc `Step` instances to be the target of the
documentation rather than the target of the rustdoc/tool being run.
This should enable `make dist` to start producing a bunch of `rust-docs`
packages for all the cross architectures that rustbuild is producing now.
This commit implements the `make dist` command in the new rustbuild build
system, porting over `dist.mk` and `prepare.mk` into Rust. There's a huge amount
of complexity between those two files, not all of which is likely justified, so
the Rust implementation is *much* smaller.
Currently the implementation still shells out to rust-installer as well as some
python scripts, but ideally we'd rewrite it all in the future to not shell out
and be in Rust proper.
