Fix the handling of the `CUcheckpointRestoreArgs_st` struct

[leofang]

This seems very complicated. I don't think we ever have code that does conditional logic for struct members? Is it possible to keep it simple, to make it easier to reason about what code would be eventually used to generate a binary?

1. cython-gen does not use megaheader, so whether or not we expose the complete member list in a cdef extern from block does not matter, the C compiler will do the right job
2. the tempita + pyclibrary template approach will be killed soon-ish

Originally posted by @leofang in #2144 (comment)