mk: Update how the build deals with version labels. #16677

Adds a new configure flag, --release-channel, which determines how the version
number should be augmented with a release label, as well as how the distribution
artifacts will be named. This is entirely for use by the build automation.

--release-channel can be either 'source', 'nightly', 'beta', or 'stable'.

Here's a summary of the affect of these values on version number and
artifact naming, respectively:

* source - '0.12.0-pre', 'rust-0.12.0-pre-...'
* nightly - '0.12.0-nightly', 'rust-nightly-...'
* beta - '0.12.0-beta', 'rust-beta-...'
* stable - '0.12.0', 'rust-0.12.0-...'

Per http://discuss.rust-lang.org/t/rfc-impending-changes-to-the-release-process/508/1

The only immediate practical impact of this is that nightlies will now be versioned `0.12.0-nightly` instead of `0.12.0-pre-nightly`.

r? @alexcrichton