Loongson MMI: Merge with MIPS64/add auto-detection Modern Loongson processors are MIPS64-compatible, and MMI instructions are now supported in the mainline of GCC. Thus, this commit adds compile-time and run-time auto-detection of MMI instructions and moves the MMI SIMD extensions for libjpeg-turbo from simd/loongson/ to simd/mips64/. That will allow MMI and MSA instructions to co-exist in the same build once #377 has been integrated. Based on: https://github.com/FlyGoat/libjpeg-turbo/commit/82953ddd61549428f58066f7eff0d60ce7429865 Closes #383