Prevent setting an undeclared attribute in Flags.

[sklam]

Prevent problem like this: #9539 (review)

[gmarkall]

gpuci run tests

[gmarkall]

I have a strange recollection that this is somehow used for something somewhere, but I don't recall what - just running gpuCI in case the CUDA target is somehow overloading flags. It may be that a Flags instance gets created from a CUDAFlags at some point when overriding the target or something like that.

[gmarkall]

Looks good to me. I was concerned about this creating issues for target extensions, for example with my changes in #8773 and some follow-on changes I'd like to make, but I think we should merge this because it makes flag handling more robust, and if I run into an issue then I should propose a principled way to extend flags.

[gmarkall]

I would mark this RTM but I don't want to cause any confusion before before the branch for 0.60, so I'll mark it RTM once we've branched.