x86_64 is used as a baseline: the "x86_64" entry, whatever status it has,
is transformed into "all", and then the other entries in ARCHITECTURES
either dropped or rearranged appropriately.
The recipe for 1.9.0 will build the full compiler suite for x86_gcc2,
and it will 'wrap' a binary stage0 compiler for x86_64.
The recipe for 1.10.0 is a complete build.
These recipes don't create the rust_runtime packages. This package
should be recreated at a later point, but we will need to think about
the package naming, as it will be very much possible to have various
runtime libraries running alongside each other.
