OpenBSD: add signal-stack machine context struct on x86_64.

[cfallin]

The ucontext_t struct on OpenBSD (which is actually a typedef to
struct sigcontext) represents the on-stack saved state that is
available for a signal handler on OpenBSD to read and/or modify. Its
definition comes from <machine/signal.h>, and was copied from OpenBSD
6.9, which is the current version. Note that OpenBSD does not guarantee
ABI compatibility across versions, so this would have to be updated if
the frame format ever does change; however, it is very useful to have
(it is not possible to know the faulting PC in a signal handler
otherwise), so I suspect including this definition is worthwhile.

[rust-highfive]

Some changes occurred in OpenBSD module

cc @semarie

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @JohnTitor (or someone else) soon.

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[semarie]

there is already on-going work in #2189 to add sigcontext_t definition. thanks.

[cfallin]

Ah, that's fantastic, thanks! I hadn't seen that PR, sorry. Will close this PR now.