Shorten toolchain names for display when they correspond to the default architecture

Here's an example of current output:

info: syncing channel updates for 'stable-x86_64-unknown-linux-gnu'
info: syncing channel updates for 'beta-x86_64-unknown-linux-gnu'
info: syncing channel updates for 'nightly-x86_64-unknown-linux-gnu'
info: downloading component 'rustc'
info: downloading component 'rust-std'
info: downloading component 'rust-docs'
info: downloading component 'cargo'
info: installing component 'rustc'
info: installing component 'rust-std'
info: installing component 'rust-docs'
info: installing component 'cargo'
info: syncing channel updates for '1.8.0-x86_64-unknown-linux-gnu'
info: checking for self-updates
info: rustup is up to date

  stable-x86_64-unknown-linux-gnu unchanged - rustc 1.8.0 (db2939409 2016-04-11)
    beta-x86_64-unknown-linux-gnu unchanged - rustc 1.9.0-beta.1 (37a2869af 2016-04-12)
   nightly-x86_64-unknown-linux-gnu updated - rustc 1.10.0-nightly (a0c325980 2016-04-20)
   1.8.0-x86_64-unknown-linux-gnu unchanged - rustc 1.8.0 (db2939409 2016-04-11)

This is too verbose for my taste. Since these are all for the default architucture I'd rather they not mention it.

For platforms with multiple viable architectures like windows it may still be useful to indicate the target triple, so this information may be important to indicate somewhere.

I haven't convinced myself that I actually want to do this, but asked for feedback on irlo.

No files or tests are named. Start by locating the CLI output that formats toolchain names and determine how the default architecture is identified. Done means default-architecture toolchains display without their target triple, while platforms where the target matters retain enough target information.