Rationale: previously we had archs on toplevel, e.g. out/x86_64
However, host tools like QEMU and gem5 can reuse a lot of the common build
files across archs. Therefore, we save space and time by putting them into
a single directory.
Therefore, the toplevel out/x86_64 was inconsistent, better put arch
inside guest tools that need separate build trees instead, e.g.
out/buildroot/x86_64/
Also common was pretty obscure as a name to say the best.
