builddeb: Add automatic support for riscv* architectures

Debian currently only defines "riscv64", but it seems safe to assume
that any 32-bit port will now be called "riscv32", also matching
$UTS_MACHINE.

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