[openacc] adjust default num_gangs

[Cesar Philippidis]

This patch teaches the libgomp runtime how to probe the CUDA driver to
extract the number of Stream Multiprocessors that are available on the
graphics hardware and use that as the default value for num_gangs.
Without that patch, libgomp used to have num_gangs default to 32, which
was chosen arbitrarily. At least this value maps onto a hardware value.

More details regarding this patch can be found here:

  https://gcc.gnu.org/ml/gcc-patches/2016-08/msg02064.html
  https://gcc.gnu.org/ml/gcc-patches/2016-08/msg02084.html

Is this patch OK for trunk?

Cesar
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