[ARM] noMMU: block sys_fork in nommu mode

The sys_fork is not supported in nommu mode. The other syscalls
that is not supported in nommu mode are to be defined as cond_signal
in kernel/sys_ni.c.

Signed-off-by: Hyok S. Choi <hyok.choi@samsung.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>