loop-unroll - add function callback argument for outside targets to

provide unroll factors, and a cmd line argument to specify number of
innermost loop unroll repetitions.

- add function callback parameter for outside targets to provide unroll factors
- add a cmd line parameter to repeatedly apply innermost loop unroll a certain
  number of times (to avoid using -loop-unroll -loop-unroll ...; instead
  -unroll-num-reps=2).
- implement the callback for a target
- update test cases / usage

PiperOrigin-RevId: 225843191