[starnix][selinux] Improve ExtensibleBitmap Iterator

Avoid iterating through the bitset bit by bit by using
trailing_zeros/ones to count the bits using a single instruction.

Bug: 505002667
Change-Id: Ia687708a5989cad1b5015cf10c925b526a6a6964
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1635198
Fuchsia-Auto-Submit: Gwen Mittertreiner <gmtr@google.com>
Reviewed-by: Wez <wez@google.com>
Commit-Queue: Gwen Mittertreiner <gmtr@google.com>