std::unix::thread fix available_parallelism on netbsd/32 bits.

[devnexen]

when bootstrapping rust, it segfaults in i386, ppc, arm ... there is no cpu attached by default in such early stage.
therefore removing the affinity api and falls back on the always reliable sysctl/HW_NCPU call instead.

[rustbot]

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[Amanieu]

Can you explain why this would segfault? Can't we just fall back if pthread_getaffinity_np returns an empty set?

[devnexen]

it segfaults at

match libc::_cpuset_isset(i, set)

which might indicate a system issue if pthread_getaffinity_np returns success ... but it does crash on 32 bits.
so for simplicity sake, instead of enabling it only on 64 bits, I propose to remove wdyt ?

[Amanieu]

I would prefer if this was fixed to work properly, but I can also accept just removing it.

[Amanieu]

Could you debug the issue on netbsd to figure out why it is crashing? We are using the C API correctly as far as I can tell. Maybe the libc bindings are incorrect?

[JohnCSimon]

@devnexen
ping from triage - can you post your status on this PR? This PR has not received an update in a few months.

[devnexen]

Apologies I got sidetracked in the meantime. After updating the platforms, I could no longer reproduce.