Sort out the various machine type names

We're using four different ways to define the achitecture. There is the
machine name as given by e.g. uname -m, the dpkg architectures (armhf,
amd64, etc) and finally the gnu arch triplet (e.g. aarch64-linux-gnu).
Have TargetTriplet recognize all three and properties for all three
variants.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D5253