Add initial AVX512 support for dot product on Linux (#320)

* Update Makefile to detect AVX512 support and add compiler flags if it's available
 * Based on existing AVX2 implementation, dot product on one 32-value block of 4-bit quantized ints at a time
 * Perform 8 bit -> 16 bit sign extension and multiply+add on 32 values at time instead of 16
 * Use built-in AVX512 horizontal reduce add to get sum at the end
 * Manual unrolling on inner dot product loop to reduce loop counter overhead