This makes usage of the MachineOperatorBuilder more robust, as it will be
an error to request an unsupported operator.

Along the way, I noticed that all 7 platforms support Float32Abs and
Float64Abs. Should make them non-optional in another CL?

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1128133003

Cr-Commit-Position: refs/heads/master@{#29223}