next-cube.c: move static old_scr2 variable to NeXTPC

next-cube.c: move static old_scr2 variable to NeXTPC

Move the old_scr2 variable to NeXTPC so that the old SCR2 register state is
stored along with the current SCR2 state.

Since the SCR2 register is 32-bits wide, convert old_scr2 to uint32_t and
update the SCR2 register access code to allow unaligned writes.

Note that this is a migration break, but as nothing will currently boot then
we do not need to worry about this now.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Thomas Huth <huth@tuxfamily.org>
Message-ID: <20231220131641.592826-9-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>