Fixes for 32-bit SPARC on Linux This PR fixes a number of issues which previously prevented `rustc` from being built successfully for 32-bit SPARC using the `sparc-unknown-linux-gnu` triplet. In particular, it adds linking against `libatomic` where necessary, uses portable `AtomicU64` for `rustc_data_structures` and rewrites the spec for `sparc_unknown_linux_gnu` to use `TargetOptions` and replaces the previously used `-mv8plus` with the more portable `-mcpu=v9 -m32`. To make `rustc` build successfully, support for 32-bit SPARC needs to be added to the `object` crate as well as the `nix` crate which I will be sending out later as well. r? nagisa