sh: Add a simple code dumper for SUPERH32 show_regs().

This implements a simple show_code() that is in turn plugged in to
show_regs() to provide minimal code dumping at the end of the trace.

Built on top of a simple instruction disassembler derived from the
binutils opcode table.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>