The `xmipscmove` is just the name of the extension. The 'mips' here means MIPS-the-company, and not MIPS-the-ISA. It's supported by the MIPS P8700 CPU which, counterintuitively, is a RISC-V CPU and not MIPS (it's named "MIPS" because the company which designed it is called "MIPS", not because it uses the MIPS architecture).