> on the Ubuntu side focal is using 9.3. Is that gcc version even affected?
Yes, we have backported the outline-atomics flag to Ubuntu 20.04 gcc-9.3.
On arm64 you can check whether a compiler has the option with:
$ gcc -v --help -Q 2>/dev/null | grep outline-atomics
-moutline-atomics [disabled]
$ gcc --version
gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
[...]
Sebastian