Linkage regression in servo with 1.2

[brson]

See servo/servo#6591 (comment)

After some tweaks to how rustc drives the linker Servo no longer links. We should decide whether this is intentional and acceptable.

[brson]

cc @pcwalton @larsbergstrom

[larsbergstrom]

This is particularly painful for us because it disallows multiple references to the same library. We had to use those multiple references in order to force a deterministic link order on platforms where symbol availability is based on the order in which libraries are listed.

I assume that Rust and Cargo now provide enough control that we should be able to remove that usage pattern now, but this change had some pretty far-rippling effects for us, even to fix it on just OSX and just the case where we're producing a shared library.

[alexcrichton]

I think Servo's now working after upgrading and otherwise I've transferred tags to #26869 (the PR in question)