add more /proc/cpuinfo tests

[gnzlbg]

@randombit has a huge corpus of cpuinfo files for all the architectures we currently care about. We should figure out a way to test the run-time feature detection system against that.

[marshallpierce]

I did some work on ring's runtime detection with cpuinfo, so we could lift from that: https://github.com/briansmith/ring/pull/392/files#diff-43684ec91e5967978239dce34e7beeb7

That ended up not being used in ring (decided to not bother with cpuinfo because the platforms in question could have capabilities queried in other ways) but it might save a few minutes of implementation. :)