ARC: build: Use __force to suppress per-CPU cmpxchg warnings

Currently, the cast of the first argument to cmpxchg_emu_u8() drops the
__percpu address-space designator, which results in sparse complaints
when applying cmpxchg() to per-CPU variables in ARC.  Therefore, use
__force to suppress these complaints, given that this does not pertain
to cmpxchg() semantics, which are plently well-defined on variables in
general, whether per-CPU or otherwise.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202409251336.ToC0TvWB-lkp@intel.com/


Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: <linux-snps-arc@lists.infradead.org>
Signed-off-by: Vineet Gupta <vgupta@kernel.org>