dsp/cpu: add AVX2 detection

currently unused.
https://software.intel.com/en-us/articles/how-to-detect-new-instruction-support-in-the-4th-generation-intel-core-processor-family
http://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-optimization-manual.pdf

Change-Id: I314200f890c58b9a587b902b214f90deb95f0579