sh: Share exception vector table for SH-3/4.

The only difference between these at the moment are the FPU
exceptions, and these are hidden away under CONFIG_SH_FPU
(which is only set for the SH-4 case anyways..).

This consolidates the two tables, and updates SH-4 to use
the updated copy.

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