LoongArch: BPF: Support unconditional bswap instructions

Add support for unconditional bswap instruction. Since LoongArch is
always little-endian, just treat unconditional bswap the same as big-
endian conversion.

Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>