task: Add missing write-barrier in `save_stack` (#62387)
Reproduces readily with `JULIA_COPY_STACKS=1` in an
`MMTK_PLAN=StickyImmix` build:
```julia
$ JULIA_COPY_STACKS=1 ./julia --startup-file=no -e "fib(x) = @sync begin; function f(x); x in (0, 1) && return x; a = Threads.@spawn f(x - 2); b = Threads.@spawn f(x - 1); fetch(a) + fetch(b); end; f(x); end; println(fib(25))"
[2035119] signal 11 (1): Segmentation fault
```
This doesn't affect the stock GC which has a persistent remset, but it
does matter for MMTK.
Investigated from core dump by Claude Fable 🤖 (w/ some human prodding)
