This was clobbering the BMI2 boolean unintentionally.
Currently unused but at least detect the feature so that our Arm64 JIT can use it in the future.
It is not an external component, and it makes paths needlessly long. Ryan seemed amenable to this when we discussed on IRC earlier. Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>