netbsd adding mcontext related data for riscv64

[devnexen]

[rustbot]

r? @JohnTitor

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[bors]

:umbrella: The latest upstream changes (presumably #3525) made this pull request unmergeable. Please resolve the merge conflicts.

[JohnTitor]

Could you resolve conflicts and remove libc_union cfg?

[tgross35]

This probably needs a rebase at this point, but otherwise seems like it was waiting for review.

@rustbot review

[tgross35]

Can this just be moved to netbsd/mod.rs? I'm not sure what is platform-specific here, if you have a header link that would help.

This should also get the semver test

[devnexen]

well ucontext could possibly be make common but not mcontext which is very architecture dependent.