Spurious ld.exe failures on MinGW

[alexcrichton]

First discovered on https://ci.appveyor.com/project/rust-lang/rust/build/1.0.2609/job/bjehestg1lwq0smv, this is likely related to 4.9.3 -> 6.2.0 upgrade

[alexcrichton]

Actually this only shows up on #40541, so I don't think it's a spurious failure.