builddeb: Add automatic support for ppc64 and powerpcspe architectures

We currently label 64-bit big-endian kernel packages as
powerpc (32-bit), mostly because it was officially supported while
ppc64 (64-bit big-endian) was not.  Now neither is officially
supported, so label these packages as ppc64.

Debian also has a powerpcspe (32-bit with SPE) architecture.
Label packages with a suitable configuration as powerpcspe.

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