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.
Both point to the same path for cc and cxx, but not for gcc_dir, which
makes no sense. That's the only significant difference between both, so
just merge them both, and use the merged file in the taskcluster build
script.
