[og7] Backport PR74048 and PR81352 nvptx fixes

[Cesar Philippidis]

I've applied this patch to openacc-gcc-7-branch which backports Tom's
nvptx bug fixes for PR74048 and PR81352. For some reason, I thought the
test case for pr84028 was triggering a new PTX JIT bug. But it turned
out to be failing because og7 is more verbose than trunk when it detects
unused parallelism (in this case, num_workers was being set but there
was no worker partitioned loop). That problem went away with an extra
dg-warning line.

Cesar
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