It appears that at the moment the only supported toolchain is r23 (see https://developer.android.com/ndk/downloads). Everything else has been marked as “unsupported”.
However, building Syncthing with the r23 toolchain using even the newest Go 1.17 results in
ld: error: cannot open output file $WORK\b083\_cgo_.o: Function not implemented
clang: error: linker command failed with exit code 1 (use -v to see invocation)
exit status 2
exit status 1
Does this mean that the only way to build Syncthing for Android is to keep using the old r21e toolchain?