Switch cmake build to Ninja
Switch cmake build to Ninja

This gets us core-dependent parallelism for free.

Closes https://github.com/google/iree/pull/489

Closes https://github.com/google/iree/pull/510

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/iree/pull/510 from GMNGeoffrey:ninja 9ea39edc6d0d4a70b75880cc52797c8234c0745c
PiperOrigin-RevId: 290329106

google/iree,iree-org/iree,iree-org/iree,google/iree,iree-org/iree,google/iree,iree-org/iree,google/iree,google/iree,iree-org/iree,iree-org/iree,google/iree,iree-org/iree,google/iree