Support custom constraints in toolchains

Bazel does not supply platforms for many architectures, specifically bare metal.

Therefore, based on the existing `triple_to_constraint_set` logic, we cannot parse things that do not have natively defined platforms within Bazel which relies on Rust and Bazel being in sync in an uncomfortable way.

My current solution to this will be implementing an override for the constraint/exec sets so that users can customize the platforms. It also solves #206 for me since I can just pass the target triple through to rustc and ignore all of the parsing.