As an alternative to changing the JIT to produce SWPAL/CASAL, because I 
wasn't sure if *all* aarch64 targets support these, I tried adding DMB 
ISH or DMB SY to the end of the generated code sequences. Surprisingly, 
this did not fix the issue! So there's perhaps something fishy about the 
LDAXR-STLXR sequences themselves?

So for now I'll stick on my own machine with SWPAL/CASAL, since this 
does seem to work well enough to let both my own code and fibers run.

Tony