Fix two bugs in LoongArch sleigh code

[jiegec]

Add missing csr77 to LoongArch, and fix csrxchg definition: The CSRXCHG instruction writes the old value of the general register rd to the bits of the specified CSR corresponding to the write mask of 1 according to the write mask information stored in the general register rj. The rest of the bits in the CSR remain unchanged, and the old value of the CSR is updated to the general register rd..

[GhidorahRex]

That missing register needs to get fixed. I'll try to get this put in asap.