builddeb: Add automatic support for mips{,64}r6{,el} architectures

MIPS R6 is not fully backward-compatible, so Debian has separate
architecture names for userland built for R6.  Label kernel
packages accordingly.

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