fix AVX & AVX2 detection

fixes issue #790 which resulted in a SIGILL on OpenBSD

code is mostly from libwebp, based on the following:

https://software.intel.com/en-us/articles/how-to-detect-new-instruction-support-in-the-4th-generation-intel-core-processor-family

Change-Id: Ida7c1a18261e98c05ed9c662068140be407ec107
1 file changed