ARC: arch_cmpxchg_relaxed to use size of pointed type not pointer

Bradley spotted this snafu when working on two-byte cmpxchg emulation.
He originally folded this fix into his series but the fix itself
uncovered a pre-existing issue, missing arch_atomic64_cmpxchg_relaxed
which just needed to be wired up.

Fixes: e188f3330a13 ("ARC: cmpxchg/xchg: rewrite as macros to make type safe")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202609240539.AkulvjVe-lkp@intel.com/
Suggested-by: Bradley Morgan <brads@mainlining.org>
Signed-off-by: Vineet Gupta <vgupta@kernel.org>