unwind-libdw: fix initialization of libdwfl cache

This fixes stack trace printing for early syscalls that precede
the first syscall from memory mapping or execve families.

* unwind-libdw.c (tcb_init): Set struct ctx.last_proc_updating
to a value different from mapping_generation so that libdwfl cache
is properly initialized before the first use.
* NEWS: Mention this fix.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1788636