rustc_trans: fix small aggregate returns for big-endian mips64 FFI

[draganmladjenovic]

Current model of threating small aggregate returns as smallest encompassing integer works only for little-endian mips64.
The patch forces small aggregate return values to be viewed as one or two i64 chunks leaving to the casting implementation
to handle endianes differences.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nikomatsakis (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 Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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.

[sanxiyn]

Don't we test this? See src/test/run-pass/extern-return-*.rs.

[draganmladjenovic]

@sanxiyn Yes, sorry for not including that in the message. Those tests are failing for me locally.

[sanxiyn]

If those tests failed before this PR and now pass after this PR, that's good enough for me. @bors r+

[bors]

📌 Commit 19b5113 has been approved by sanxiyn