builddeb: Add automatic support for sh{3,4}{,eb} architectures

Different generations of the SH architecture are not very compatible,
so there are/were separate Debian ports for SH3 and SH4.

Move the fallback out of the "case" statement, so that it will also be
used in case we find some SH architecture version without a known
mapping.

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