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 rustc_llvm.
FIXME: Split this into two crates: one that has deps on syntax, and one that doesn't; the one that doesn't might get decent parallel build speedups.