opcodes: LoongArch: add "ret" instruction to reduce typing

This syntactic sugar is present in both classical and emerging
architectures, like Alpha, SPARC and RISC-V, and assembler macros
doing the same thing can already be found in the wild e.g. [1], proving
the feature's popularity. It's better to provide support directly in the
assembler so downstream users wouldn't have to re-invent this over and
over again.

[1]: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/loongarch/sysdep.h;h=c586df819cd90;hb=HEAD#l28