LoongArch: Add atomic operations for 32BIT/64BIT

LoongArch: Add atomic operations for 32BIT/64BIT

LoongArch64 has both AMO and LL/SC instructions, while LoongArch32 only
has LL/SC intstructions. So we add a Kconfig option CPU_HAS_AMO here and
implement atomic operations (also including local operations and percpu
operations) for both 32BIT and 64BIT platforms.

Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>