[OpenMP] libomp: fix dynamic loop dispatcher

Restructured dynamic loop dispatcher code.
Fixed use of dispatch buffers for nonmonotonic dynamic (static_steal) schedule:
- eliminated possibility of stealing iterations of the wrong loop when victim
  thread changed its buffer to work on another loop;
- fixed race when victim thread changed its buffer to work in nested parallel;
- eliminated "static" property of the schedule, that is now a single thread can
  execute whole loop.

Differential Revision: https://reviews.llvm.org/D103648