Some stuff used by rustc that doesn't have many dependencies
Originally extracted from rustc::back, which was nominally the
compiler 'backend', though LLVM is rustc's backend, so rustc_target
is really just odds-and-ends relating to code gen and linking.
This crate mostly exists to make rustc smaller, so we might put
more 'stuff' here in the future. It does not have a dependency on
LLVM.