sh: Fix DSP opcode regression for SH3-DSP parts.

Older versions of binutils do not support -Wa,-isa= tuning, which is
something we rely on for enabling DSP opcode support on the newer
parts. SH3-DSP parts can still be handled with -Wa,-dsp even if the
newer parts require the newer versions of binutils for supporting the
new opcodes.

This was broken in -rc1 when the SH4AL-DSP support was being reworked,
and is needed to get SH3-DSP working with older toolchains again.

Reported-by: Markus Brunner <super.firetwister@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>