[bmi2] Add lzcnt and bmi target attributes

[bmi2] Add lzcnt and bmi target attributes

* When dynamic dispatching to bmi2 add lzcnt and bmi to the
  TARGET_ATTRIBUTE.
* Centralize the bmi2 TARGET_ATTRIBUTE definition to
  BMI2_TARGET_ATTRIBUTE so we can change it in the future.
* Only enable bmi2 when both bmi1 & bmi2 are supported. There shouldn't
  be any cases where bmi2 is supported but bmi1 isn't. But, since we are
  using the instruction we should check bmi1 as well.