[gomp4] More deferral of partitioning to target

[Nathan Sidwell]

I've committed this to  gomp4 branch.

It is another step towards deferring  partitioned execution choices to the 
target compiler -- though sadly not the last step.

At early omp lowering, we now attach partitioning flags to the HEAD_MARK 
function I introduced yesterday, and adjust the partition enter/exit sequence to 
not be level specific.

At the oacc_device_lower stage, the loop structure is augmented to hold up to 3 
levels of head & tail markers, allowing a single loop to be partitioned over up 
to 3 axes.  Once constructed we  then iterate of the marker sequences replacing 
the dummy axis argument of the appropriate builtins with the specific chosen axis.

The next step is to iterate over the body doing the same for the loop 
abstraction builtin.

nathan
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