Only allow whitelisted cpu flags by default

[bchurchill]

We should only ever use a cpu flag if we have tested it and know it works properly. New architectures add new flags, and bringing them in by default is problematic. I believe this is the root cause for #929 and #840. I think for now we should just keep these:

avx avx2
sse sse2 ssse3 sse4_1 sse4_2
fma cmov popcnt
bmi1 bmi2

Opinions? This should be nearly a one-liner.

[stefanheule]

Are you proposing to change the default cpu flags to only the list we have tested? If so, sounds good to me.

[bchurchill]

Yup. Ok, I'll do this when I get the chance.