[patch,openacc] Check clauses with intrinsic function specified in !$ACC ROUTINE ( NAME )

[Cesar Philippidis]

This patch allows Fortran intrinsic functions to be declared as acc
routines. For instance, abort can now be called from acc within
offloaded regions.

Given that intrinsic functions like sin and cos are important for
offloaded functions, I wonder if there is a better way to accomplish to
enabling this. Maybe certain intrinsic functions should default to
having an implied acc routine directive. But I suppose that's something
for another patch.

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

Thanks,
Cesar