sh: Simplify and lock down the ISA tuning.

The ISA tuning as it is today can not cope with all of the different
variations that are possible, so all we can do is a best attempt based on
the CPU family. The DSP and FPU generation are already at odds with each
other, and the nommu tuning we weren't handling at all.  Additionally,
for platforms that never had an FPU, the -nofpu variant never existed,
meaning that we would lose out on family granular tuning completely in
certain cases.

With tat out of the way, we were also using -up versions, allowing for
later instructions that branched off of a particular subset of the ISA,
but are not actually reflected on the hardware being targetted. This
leads to some confusion, and the possibility of bogus instructions on
older parts. Kill that off and lock it down to the family being built
for specifically.

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