LoongArch: Add some instruction opcodes and formats

According to the "Table of Instruction Encoding" in LoongArch Reference
Manual [1], add some instruction opcodes and formats which are used in
the BPF JIT for LoongArch.

[1] https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html#table-of-instruction-encoding

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>