SPARC: optimized interrupt trap stores and loads

The input registers (i0..i7) are not modified by the interrupt trap
handler and are preserved by function calls. So we do not need to
store them in the interrupt stack frame.

This saves 48 bytes of stack space for each interrupted context,
and eliminates 4 double word stores and 4 double word loads.

Signed-off-by: Martin Åberg <martin.aberg@gaisler.com>