Add FreeBSD10 support

[gnzlbg]

This adds libc-test support for Freebsd10 and a CI build job that tests
FreeBSD10 with LIBC_CI only.

cc @asomers this is a follow up to #1491 .

[rust-highfive]

@gnzlbg: no appropriate reviewer found, use r? to override

[asomers]

I still disagree with this PR on general principle: FreeBSD 10 is EOL so we shouldn't bother to support it anymore. I assume that reason to do it is that libstd still uses a FreeBSD 10 build host? If so, can we get them to upgrade their builder instead?

[gnzlbg]

I assume that reason to do it is that libstd still uses a FreeBSD 10 build host?

libstd uses a FreeBSD10 build host, yes.

If so, can we get them to upgrade their builder instead?

Maybe someone can, but the only thing I can do is try to prevent libc from being broken on FreeBSD10 hosts until libstd migrates away from it so that broken libc's don't end up in libstd, and as a result all FreeBSD hosts end up broken.

[asomers]

Does the current situation result in libstd that is broken on FreeBSD 11 and 12 hosts, or only broken on FreeBSD 10 hosts?

[gnzlbg]

Does the current situation result in libstd that is broken on FreeBSD 11 and 12 hosts, or only broken on FreeBSD 10 hosts?

The situation that happened a couple of weeks ago broke rustc on FreeBSD10 and 11 hosts. I think it was a consequence of us not testing FreeBSD10, because the libc/build.rs was broken for that host, and ended up picking up freebsd12 symbols, and that resulted in the broken rustc.

[gnzlbg]

So I'm not sure what to do about this. If there isn't consensus then I think I'll just close it.

[gnzlbg]

I'm merging this. If someone wants to put in the work of upgrading FreeBSD in rust-lang/rust upstream, they can do so, and let us know, and then I'll remove all of this. But until that happens, we should try to make sure that rust-lang/rust doesn't generate broken toolchains for FreeBSD targets.

@bors: r+

[bors]

📌 Commit 3843c7d has been approved by gnzlbg

[bors]

⌛ Testing commit 3843c7d with merge f66ba4f...

[bors]

💡 This pull request was already approved, no need to approve it again.

• This pull request is currently being tested. If there's no response from the continuous integration service, you may use retry to trigger a build again.

[bors]

📌 Commit 3843c7d has been approved by gnzlbg

[bors]

⌛ Testing commit 3843c7d with merge 58b2706...

[bors]

☀️ Test successful - checks-cirrus-freebsd-11, checks-cirrus-freebsd-12, status-azure
Approved by: gnzlbg
Pushing 58b2706 to master...