ARC: syscall for userspace cmpxchg assist

Older ARC700 cores (ARC750 specifically) lack instructions to implement
atomic r-w-w. This is problematic for userspace libraries such as NPTL
which need atomic primitives. So enable them by providing kernel assist.
This is costly but really the only sane soluton (othern than tight
spinning using the otherwise availiable atomic exchange EX instruciton).

Good thing is there are only a few of these cores running Linux out in
the wild.

This only works on UP systems.

Reviewed-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>