sh: Re-add support for best fit ISA tuning if none is available.

This was removed in the libgcc integration, but there are still some
compilers that need this. We also relax the rules on the ISA tuning in
the cases where there are no matches for the CPU tuning and adopt the
-any default, which matches the intent of the isa-y target list. This
compensates for mismatches where binutils supports a wide array of
targets whilst the compiler is much more restricted.

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