84d6e8aed3
Add an `abi` field to `TargetOptions`, defaulting to "". Support using `cfg(target_abi = "...")` for conditional compilation on that field. Gated by `feature(cfg_target_abi)`. Add a test for `target_abi`, and a test for the feature gate. Add `target_abi` to tidy as a platform-specific cfg. This does not add an abi to any existing target.