In tests/Makefile, add ARCHS and BUILD_VIA variables to allow
selecting a set of architectures to build via gcc or guix and test via
QEMU. For example:
make ARCHS="amd64 riscv" BUILD_VIA=qemu
where BUILD_VIA defaults to gcc, expecting you to have installed all
the relevant gcc variants.
Add arch-name and build-via adapters to handle translating between
GCC, QEMU, and Guix (architecture names, etc.).
See the comments in tests/Makefile for more information.