builddeb: Add automatic support for sparc64 architecture

We currently label 64-bit kernel packages as sparc (32-bit), mostly
because it was officially supported while sparc64 was not.  Now
neither is officially supported, so label these packages as sparc64.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>