Improve CPU architecture tolerance (#1142)
Improve CPU architecture tolerance (#1142)

numpy's CPU info dictionary does not have a 'flags' field on some
CPU architectures (such as ppc64le). Tolerate absence of 'flags'
when checking for instruction set to allow faiss to continue in
non-AVX2 mode on those architectures.
facebookresearch/faiss,facebookresearch/faiss,facebookresearch/faiss,facebookresearch/faiss