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:
82953ddd61Closes#383
