Add support for 3 new architectures: hexagon, openrisc, and um

This MR also adds support for overriding the container image at the architecture level. For example, there is no proper crossbuild image for hexagon or um, but the former can be built with clang so we can use the plain tuxmake/clang* images; the later can be built using any x86_64 images.

We also expand on the architecture support documentation, detailing which architectures have full support for container builds (i.e. kernel + userspace tools), which can build the kernel only, and which are not supported at all.

Merge requests !152 (closed) and !153 (closed) are included in this one.