builddeb: Add automatic support for mips64el architecture

We currently label 64-bit little-endian kernel packages as
mipsel (32-bit little-endian), mostly it was officially supported
while mips64el (64-bit little-endian) was not.  Now both are
officially supported, so label these packages as mips64el.

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