[patch][OpenACC] Add target hook TARGET_GOACC_ADJUST_PARALLELISM

[Cesar Philippidis]

At present, GCC fixes the vector length on all targets. However, that is
an artificial restriction. This patch introduces a new
TARGET_GOACC_ADJUST_PARALLELISM hook that enables the runtime to correct
the default number of acc workers and vectors. Extra care need to be
done to ensure that large vectors fit inside workers. The target hook
itself doesn't do anything for the host, but the nvptx BE will make use
of it.

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

Thanks,
Cesar
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