cmd/link: external linking is not reproducible on some platforms

[mdempsky]

I've noticed that running go build -a -v -toolexec='toolstash -cmp' std cmd fails on ppc64le, because some binaries (e.g., cmd/pprof) report differences.

I haven't investigated much yet, but as a hunch I suspect it's related to ppc64le not supporting internal linking for cgo. E.g., cmd/pprof depends on net (which uses cgo) and fails, but cmd/gofmt passes. Also, I'm not able to reproduce the issue when cross-building either ppc64le->amd64 or amd64->ppc64le (where both modes disable cgo), or if I build natively on ppc64le with CGO_ENABLED=0.

Notably though, go build -ldflags=-linkmode=external -toolexec='toolstash -cmp' cmd/pprof does not fail on amd64. So maybe the non-determinism is specific to the external linker used on my ppc64le system vs my amd64 system.

Happy to run additional experiments to help narrow down the cause of failure.

My ppc64le system is running Fedora 34, with GCC 11.0.1 and GNU ld 2.35.1-41.fc34. The amd64 test system is a Google corp workstation.

/cc @cherrymui @rsc

[cherrymui]

Yeah, see also CL https://go-review.googlesource.com/c/go/+/253157 . That fails on a few platforms with external linking (including PPC64, if I recall correctly). In the past I have fixed some platforms (e.g. https://go-review.googlesource.com/c/go/+/250944 on macOS) but in general there still need more work.