The nvptx reverse-offload code mishandled the case that there was a reverse
offload function that isn't called inside a target region. In that case,
the linker did not include GOMP_target_ext and the global variable it uses.
But the plugin-nvptx.c code expected that the latter is present.

Found via sollve_vv's tests/5.0/requires/test_requires_reverse_offload.c which is
similar to the new testcase. (Albeit the 'if' and comments imply that the sollve_vv
author did not intend this.)

Solution: Handle it gracefully that the global variable does not exist - and
do this check first - and only when successful allocate dev->rev_data. If not,
deallocate rev_fn_table to disable reverse offload handling.

OK for mainline?

Tobias

PS: Admittedly, the nvptx code is not yet exercised as I still have to submit the
libgomp/target.c code handling the reverse offload (+ enabling requires reverse_offload
in plugin-nvptx.c). As it is obvious from this patch, the target.c patch is nearly but
not yet completely ready. - That patch passes the three sollve_vv testcases and also
the existing libgomp testcases, but some corner cases and more testcases are missing.
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