build: Parse the architecture field in the dsc

Debian packages can specify which architectures they are intended to
build on (e.g. any/all for any architecture) or specific architecture
specifiers to limit to certain architectures. Parse this information
such that it can be used to automatically excluded packages on
architectures for which they're not intended

Signed-off-by: Héctor Orón Martínez <hector.oron@collabora.co.uk>