MIPS: asm: local: add barriers for Loongson

Somehow these LL/SC usages are not taken care of, breaking Loongson
builds. Add the SYNCs appropriately.

Signed-off-by: Wang Xuerui <git@xen0n.name>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: <stable@vger.kernel.org> # v5.5+