Update the gecko-rust-build docker container to upload the
constructed toolchain to tooltool when running on taskcluster,
so it can be used in normal gecko builds (buildbot as well as tc)
Also include a mac build script (which can't run in taskcluster
currently) and a python script for launching the build task from
a local checkout. This is based on Ted's script for building
breakpad's minidump_stackwalk utility.
Matches contents of https://github.com/rillian/rust-build
commit 407c6854e91541341508e96e6a2781434535b7a5
tag v0.2.0
