aarch64: Fix `libunwind` init in signal handler context (#61969)
From the `unw_init_local(3)` man page:
> If the unw_context_t is known to be a signal frame (i.e., from the
> third argument in a sigaction handler on linux), unw_init_local2()
> should be used for correct initialization on some platforms,
> passing the UNW_INIT_SIGNAL_FRAME flag.
Only tested briefly but this appears to resolve
https://github.com/JuliaLang/julia/issues/61658 on AArch64 Linux for me.
(cherry picked from commit bfbe2b55da0ff0100f2b39664cccdc6e54aae4ef)