[patch,openacc] Update _OPENACC value and documentation for OpenACC 2.5

[Cesar Philippidis]

This patch formally introduces OpenACC 2.5 functionality in various GCC
documentation sources along with with updated the _OPENACC value in the
various offloading header files.

As of right now, GCC trunk already supports the updated OpenACC 2.5 data
clause semantics. Julian, Chung-Lin and I have been working on pushing
our remaining og8 patches to trunk (which we're down to under 30 now
from 170+). But a number of those changes involve performance tuning,
rather than new OpenACC functionality.

Is this patch OK for trunk? I bootstrapped and regtested it for x86_64
Linux with nvptx offloading.

Thanks,
Cesar
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