Dpkg::Arch: Make get_host_arch and get_build_arch read from the environment

Dpkg::Arch: Make get_host_arch and get_build_arch read from the environment

All scripts calling get_host_arch are expected to support getting the
value overriden by the environment when cross building. Except for
dpkg-architecture, which needs more fine grained control, and calls
directly the get_raw_ functions.