xtensa: add exclusive atomics support

Implement atomic primitives using exclusive access opcodes available in
the recent xtensa cores.
Since l32ex/s32ex don't have any memory ordering guarantees don't define
__smp_mb__before_atomic/__smp_mb__after_atomic to make them use memw.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/kernel/setup.c

@@ -650,6 +650,9 @@ c_show(struct seq_file *f, void *slot)
 #endif
 #if XCHAL_HAVE_S32C1I
 		     "s32c1i "
+#endif
+#if XCHAL_HAVE_EXCLUSIVE
+		     "exclusive "
 #endif
 		     "\n");