Enable/disable bit manipulation instruction sets based on the architecture instead of with a feature flag

[gnzlbg]

Right now the feature flag bmi2 must be explicitly passed to the library to enable the BMI2.0 instruction set. It would be better to enable this depending on the architecture.

Blocked on rust-lang/rust#30462 .

[gnzlbg]

Blocked on rust-lang/rust#34397 which would allow using -C target-feature=... or letting llc detect it based on the target architecture.

[gnzlbg]

This is handled by bitintr build.rs.