Add powerpc64-unknown-freebsd target

[pkubaj]

Per rust-lang/stdarch#765 add powerpc64-unknown-freebsd to prevent further breakages.

[rust-highfive]

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

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[gnzlbg]

Thanks for the PR, you might want to add here aarch64 as well.

FYI, this repo has nothing to do with stdsimd. My comment there was suggesting setting up a similar system as the one set up here, but on that repo, to be able to test these targets there as well.

[gnzlbg]

You need to add these targets to the RUST_NIGHTLY_LINUX_TARGETS variable, since these are only available on nightly.

[gnzlbg]

Damn, they need to be in the "LINUX_NO_CORE_TARGETS" apparently: https://github.com/rust-lang/libc/pull/1358/files#diff-78b1794b72c7105dbabbe466019395d6L170

[gnzlbg]

Thank you! Now it worked!

@bors: r+

[bors]

📌 Commit 49271b7 has been approved by gnzlbg

[bors]

⌛ Testing commit 49271b7 with merge 0b2e18f...

[bors]

💔 Test failed - checks-travis

[gnzlbg]

@bors: retry

[bors]

⌛ Testing commit 49271b7 with merge 5588425...

[bors]

💔 Test failed - checks-travis

[gnzlbg]

@bors: retry

[bors]

⌛ Testing commit 49271b7 with merge 6d4c60d...

[bors]

☀️ Test successful - checks-cirrus, checks-travis, status-appveyor
Approved by: gnzlbg
Pushing 6d4c60d to master...