sord segfaults on s390x / ppc64el - related to gcc-10.2 optimizations

Sord has issues on these platforms if built with a recent toolchain.
I reported it upstream as I got good workarounds but failed to spot the root cause.
See: https://gitlab.com/drobilla/sord/-/issues/1

The effect in Ubuntu is that packages using sort can segfault on free.
Currently this blocks https://launchpad.net/ubuntu/+source/gst-plugins-bad1.0/1.18.2-1ubuntu2
as it runs:
$ /usr/lib/s390x-linux-gnu/gstreamer1.0/gstreamer-1.0/gst-codec-info-1.0 debian/gstreamer1.0-plugins-bad/usr/lib/s390x-linux-gnu/gstreamer-1.0/libgstlv2.so

That segfaults then.
I've found that it works:
- with gcc-9
- on x86/armhf/arm64/riscv64
- with -fno-schedule-insns -fno-schedule-insns2 set

This could be either gcc-10 being too aggressive or (more likely) an issue in sord that is hard to see and not triggered up to now, but exposed by the new optimizations.
I'll file this against sord (to track the mitigation upload, as well as any discussion and point to upstream) as well as gcc-10 (since we have seen a bunch of those 'fails due to new toolchain') in case there is something in the toolchain to fix.

Steps to reproduce in Hirsute-proposed
$ sudo apt install gstreamer1.0-plugins-bad libgstreamer1.0-dev
$ /usr/lib/powerpc64le-linux-gnu/gstreamer1.0/gstreamer-1.0/gst-codec-info-1.0 /usr/lib/powerpc64le-linux-gnu/gstreamer-1.0/libgstlv2.so
gst-codec-info-1.0: ../src/zix/btree.c:716: zix_btree_get: Assertion `frame->node' failed.
Aborted (core dumped)

In the upstream bug I outlined how to (re)compile sord as needed for any experiments.
=> https://gitlab.com/drobilla/sord/-/issues/1

A Test PPA which had various iterations of trying to be surgical with the change (well, more surgical than global -O1) is at: https://launchpad.net/~ci-train-ppa-service/+archive/ubuntu/4392