[gomp4] Make OpenACC orphan gang reductions errors

[Cesar Philippidis]

This patch promotes all OpenACC gang reductions on orphan loops as
errors. Accord to the spec, orphan loops are those which are not
lexically nested inside an OpenACC parallel or kernels regions. I.e.,
acc loops inside acc routines.

At first I thought this could be a warning because the gang reduction
finalizer uses an atomic update. However, because there is no
synchronization between gangs, there is way to guarantee that reduction
will have completed once a single gang entity returns from the acc
routine call.

I've applied this patch to gomp-4_0-branch.

Cesar
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