ARM/MIPS: Change the behavior of JSIMD_FORCE* The JSIMD_FORCE* environment variables previously meant "force the use of this instruction set if it is available but others are available as well", but that did nothing on ARM platforms, since there is only ever one instruction set available. Since the ARM and MIPS CPU feature detection code is less than bulletproof, and since there is only one SIMD instruction set (currently) supported on those platforms, it makes sense for the JSIMD_FORCE* environment variables on those platforms to actually force the use of the SIMD instruction set, thus bypassing the CPU feature detection code. This addresses a concern raised in #88 whereby parsing /proc/cpuinfo didn't work within a QEMU environment. This at least provides a workaround, allowing users to force-enable or force-disable SIMD instructions for ARM and MIPS builds of libjpeg-turbo.