sparc32: drop sun4c bits from head_32.S

As we no longer support sun4c we can remove all the code to
support remapping the kernel in head_32.S.
We also try to tell the user that the machine is not support,
in case someone try to boot the kernel on a sun4c one day

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>