Constraints for ABI

Would it be possible to introduce `@platforms//abi` which contains constraints for common ABI definitions of [platform triples](https://clang.llvm.org/docs/CrossCompilation.html#target-triple)?

In `rules_rust` we currently have issues with users wanting to target platforms with shared CPU and system and unique ABI but are unable to do so as there's no constraints uniquely identifying them. Users are forced to define custom constraints and redefine toolchains to get around errors caused by this ambiguity. I think it would be very beneficial to introduce ABI constraints here so that `rules_rust` and other rules can share constraints, enabling rule maintainers to appropriately constrain toolchains so users only have to setup platform definitions.