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Ok, that cross-build did not work, at some point along the way the alsa-sys package refused to build without additional effort to make pkg-config happy for cross-compiling, probably since it links against a system library. Since I didn’t want to go there, I instead tried compiling on a rpi zero (which should compile for armv6 and also run on armv7 I hope), and compile using the Debian-supplied rustc on Buster (which I assume is also armv6 since it runs on the zero too).
On the rpi zero, I had to re-run rustup-init, since the files I previously downloaded on the rpi3 did not run (segfault). After this, the build ran fine, though it did take *long* (In total probably somewhere between 4-8 hours, at some points it was crunching on a single package for so long it seemed to be stuck, but it would just be slow it seems).
On buster, using the Rasbpian-supplied rustc package, things compiled faster (since this could run on a rpi3), but took a lot more effort to get it to work (or at least to figure out a workaround). I ran into problems compiling the typenums rust package, which I traced back to a misoptimization in the LLVM backend (see this bug I reported for details). Until that bug is fixed in Raspbian, a workaround is needed. Fortunately, it is simple, just install libllvm7 from Debian rather than Raspbian:
$ wget http://ftp.de.debian.org/debian/pool/main/l/llvm-toolchain-7/libllvm7_7.0.1-8_armhf.deb
$ sudo dpkg -i libllvm7_7.0.1-8_armhf.deb
This is only needed on the rpi that compiles librespot, not for running.
The architecture used (in both approaches) is apparently arm-unknown-linux-gnueabihf, so that’s slightly different from what I manually picked above.
Both approaches resulted in a binary that works on both zero and rpi3, and did not have the track-changing problem I started this for. The librespot compiled on the rpi zero on Stretch also worked on Buster, but the one compiled on Buster did *not* work on stretch (due to the older libc version). So, if you want to support a newer spotify plugin also on existing stretch installs, it must be compiled on older hardware to get rustup to build for the right architecture. Or maybe investigate cross-compiling after all (which might have been less work in hindsight, but I’m not going to *also* dive into that :-p).