When CONFIG_VIRT_CPU_ACCOUNTING is selected, system call exception
handler doesn't fit below 0xd00 and build fails.

As exception 0xd00 doesn't exist and is never generated by 40x,
comment it out in order to get more space for system call exception.

Fixes: 9e270862 ("powerpc/32: Warn and return ENOSYS on syscalls from kernel")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/633165d72f75b4ef4c0901aebe99d3915c93e9a2.1589043863.git.christophe.leroy@csgroup.eu