Auto merge of #4239 - Keruspe:master, r=alexcrichton

hash '__CARGO_DEFAULT_LIB_METADATA' in metadata for rustc

We already have __CARGO_DEFAULT_LIB_METADATA to force adding the hash of some metadata in the libraries file name when building rust.
For now, we only check if it is set or not.
This patch makes also use of its value to compute the hash. This way we'll be able to pass in the channel we're building to avoid hash collisions between channels.

diff --cc tests/build.rs
Simple merge