[gomp4] fortran routine backports

[Cesar Philippidis]

This patch backports the recent fortran routine support changes I've
made in trunk to gomp-4_0-branch. Nothing changed in the fortran front
end, but I corrected a couple of problems with the way that gang, worker
and vector were handled in tree-nested.c. And there's a new test case to
exercise those changes.

This patch has been applied to gomp-4_0-branch.

Cesar
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