cmd/link: -buildmode=plugin fails on linux/arm64

At least on the builder, it tickles gold the wrong way:

```
/tmp/workdir/go/pkg/tool/linux_arm64/link: running gcc failed: exit status 1
/usr/bin/ld.gold: internal error in global, at ../../gold/aarch64.cc:4973
```

Odd, as the plugin mode mostly depends on linker configuration used by either c-shared or shared.
