ade sysroot: Allow installation of amd64 sysroots

Running `ade sysroot install --release v2021dev3 --arch amd64` results
in `ade` complaining with:

    usage: ade sysroot install [-h] [--verbose] [--distro DISTRO]
                               [--release RELEASE] [--arch {armhf,arm64}]
                               [--url URL] [--user USER] [--password PASSWORD]
                               [--netrc NETRC] [--file FILE] [--force]
    ade sysroot install: error: argument --arch: invalid choice: 'amd64' (choose from 'armhf', 'arm64')

This means that developers attempting to target amd64 targets aren't
going to be able to follow the ADE documentation and we can't easily
target different releases using ADE when using an amd64 target, so
sync the list of allowable values with the main list of the kind of
sysroots we support.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>