ARC: Use __force to suppress per-CPU cmpxchg complaints

[Paul E. McKenney]

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@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-kbuild-all/202409251336.ToC0TvWB-lkp@xxxxxxxxx/
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Vineet Gupta <vgupta@xxxxxxxxxx>
Cc: <linux-snps-arc@xxxxxxxxxxxxxxxxxxx>

diff --git a/arch/arc/include/asm/cmpxchg.h b/arch/arc/include/asm/cmpxchg.h
index 58045c8983404..76f43db0890fc 100644
--- a/arch/arc/include/asm/cmpxchg.h
+++ b/arch/arc/include/asm/cmpxchg.h
@@ -48,7 +48,7 @@
 									\
 	switch(sizeof((_p_))) {						\
 	case 1:								\
-		_prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *)_p_, (uintptr_t)_o_, (uintptr_t)_n_);	\
+		_prev_ = (__typeof__(*(ptr)))cmpxchg_emu_u8((volatile u8 *__force)_p_, (uintptr_t)_o_, (uintptr_t)_n_);	\
 		break;							\
 	case 4:								\
 		_prev_ = __cmpxchg(_p_, _o_, _n_);			\