sim: sh: rework register layout with anonymous unions & structs

sim: sh: rework register layout with anonymous unions & structs

Now that we require C11, we can leverage anonymous unions & structs
to fix a long standing issue with the SH register layout.  The use
of sregs.i for sh-dsp has generated a lot of compiler warnings about
the access being out of bounds -- it only has 7 elements declared,
but code goes beyond that to reach into the fregs that follow.  But
now that we have anonymous unions, we can reduce the nested names
and have sregs cover all of these registers.