locking/atomic: remove ARCH_ATOMIC remanants

Now that gen-atomic-fallback.sh is only used to generate the arch_*
fallbacks, we don't need to also generate the non-arch_* forms, and can
removethe infrastructure this needed.

There is no change to any of the generated headers as a result of this
patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210713105253.7615-3-mark.rutland@arm.com

scripts/atomic/fallbacks/dec

@@ -1,7 +1,7 @@
 cat <<EOF
 static __always_inline ${ret}
-${arch}${atomic}_${pfx}dec${sfx}${order}(${atomic}_t *v)
+arch_${atomic}_${pfx}dec${sfx}${order}(${atomic}_t *v)
 {
-	${retstmt}${arch}${atomic}_${pfx}sub${sfx}${order}(1, v);
+	${retstmt}arch_${atomic}_${pfx}sub${sfx}${order}(1, v);
 }
 EOF