Currently, we have in-tree scripts that allow to build our toolchains.
We also have a taskcluster script for clang that can be used in with
manually created taskcluster tasks. I wrote a similar script for gcc a
while ago, for the same usage.
This change hooks all these up such that one can do a try push with
`-j linux64-clang` or `-j linux64-gcc` and get a toolchain built as a
result.
It also hooks things up such that changes to the toolchain build scripts
trigger those jobs as well, so as to ensure they are not broken by the
changes.
