If you'd like to build for a target that is not yet supported by rustc, you can use a "custom target specification" to define a target. These target specification files are JSON. To see the JSON for the host target, you can run:

$ rustc +nightly -Z unstable-options --print target-spec-json

To see it for a different target, add the --target flag:

$ rustc +nightly -Z unstable-options --target=wasm32-unknown-unknown --print target-spec-json

To use a custom target, see xargo.