feat: add qnx7.1 (io-pkt/io-sock) and qnx8 check

[lambinoo]

Following guidance in #670, I'm adding a cargo check for the main QNX targets. To avoid exploding the number of QNX related jobs, this only focuses on ARM targets. It's QNX's main target anyway, since it targets automotive. It should be enough to cover most important targets:

• io-sock QNX7.1
• io-pkt QNX7.1
• QNX8 (implicitly io-sock)

There's three commits in this PR:

1. Enable cargo check
2. to fix a rustdoc build error, adds cloexec support to QNX targets since it is supported anyway
3. Fix an oversight on my part with IPV6 multicast. Thanks cargo check! Sorry for that

I can split the other two in a different PR if it's preferred.

[Thomasdezeeuw]

Let me know when this passes the CI :)

[lambinoo]

I think it's all green (at least, as far as I can make it.., so technically not green). It's only thumbv8m.main-nuttx-eabihf that seems to be broken, and that one was failing in the other QNX PRs I think too.

Probably something that got removed from libc in the latest version?

Edit: seems to fail when it compiles libstd, so i'm guessing this is something that needs to get fixed in libc and backported into rust-lang/rust?

[Thomasdezeeuw]

Nit: can you revert this?

[lambinoo]

should be reverted!

[Thomasdezeeuw]

The CI wasn't green when I opened the page, must have been fixed before I let a comment. You can ignore nuttx.

[Thomasdezeeuw]

Thanks @lambinoo

[lambinoo]

Thanks to you too! Have a great day/evening