std_detect: Support run-time detection on OpenBSD using elf_aux_info

[brad0]

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[rustbot]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

@semarie, could you help review that this is the right way to detect platform features as the relevant target maintainer?

(per https://doc.rust-lang.org/nightly/rustc/platform-support/openbsd.html)

[semarie]

The logic seems good (but not deeply checked for now), but it shouldn't be merged at this stage.

@brad0 could you first look to make elf_aux_info function available for OpenBSD in rust libc ? else, the build will just break due to undefined function.

the steps are:

• add elf_aux_info(3) in Rust libc
• hvae it backported to 0.2 branch (should be automatic for such change)
• have a new release of libc crate with it
• update rust compiler to use this version of the crate

Once all is done, the PR could be considered.
Thanks.

[brad0]

@semarie So the libc part has been completed.