support <libc>-<kernel>-<cpu> in architecture specs

support <libc>-<kernel>-<cpu> in architecture specs

APT has a different understanding than dpkg (#748936) what matches and
what doesn't match an architecture specification as it isn't converting
back (and forward) to Debian triplets. That has to eventually be solved
some way or the other, but until that happens we change the matching in
apt so that porters can continue their work on non-gnu libc-ports even
if policy doesn't specify that yet (and dpkg just supporting it "by
accident" via triplets).

The initial patch was reformatted, fixed in terms of patterns containing
"any-any", dealing with expanding an arch without libc to gnu while a
pattern expands libc to any, the parsedepends test was fixed (the new
if's were inserted one step too early) and another test just for the
specifications added.

Closes: #812212
Thanks: Bálint Réczey for initial patch