git-update: accept any arch in arch-dependent metadata

git-update: accept any arch in arch-dependent metadata

Currently we hardcode the architectures the official repos historically
supported, which seems both inefficient because of hardcoding, and
simply wrong, because many packages support various ARM platforms too.

If we were to say "only officially supported arches will be supported in
the AUR" we'd have to disable i686, which seems silly and arbitrarily
restrictive. Also there's better places to implement such a blacklist
(via die_commit in the main loop, via a config option to list supported
arches, would make much more sense in terms of logic).

As for the metadata extraction itself, there's no reason to hardcode the
arches to check for at all. We can get this information too, from the
.SRCINFO itself. Detecting this dynamically is not incompatible with a
blacklist, should we ever decide to implement such a thing.

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>