[OpenACC 3/11] new target hook

[Nathan Sidwell]

This patch implements a new OpenACC target hook.  It is used during 
device-specific lowering and allows a backend to indicate whether it needs to 
know about a fork/join for a particular axis.

For instance, in PTX we don't care about gang-level fork and join.  We also 
don't care if the size of that dimension is 1.

The default implementation of the hook never only cares if the oacc_fork and 
oacc_join RTL  expanders exist (and they don't on the host compiler).

nathan
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