This fix was part of the submitted patch + reviewed with minor comments
patch:

[PATCH, OpenMP 5.0] More implementation of the requires directive
https://gcc.gnu.org/pipermail/gcc-patches/2021-January/563393.html

but that never got revised and applied.

Committed the most obvious parts as obvious, as
r13-800-g8af266501795dd76d05faef498dbd3472a01b305

Tobias

PS: The non-obvious parts need to be a bit revised (collect follow-up
patches, properly filter out requirements not fulfilled from the
available-devices list → cf. glossary of the current, post-5.2 OpenMP
specification git repository) etc.
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