fix dwarf-based unwinding to the end of stack (#42620)

fix dwarf-based unwinding to the end of stack (#42620)

We experience CLR crash on some architectures (at least in x86) in case
of unhandled managed exception. libunwind steps to the very end of a
stack, and if .eh_frame info is correct, it returns with retcode 0 and
ip=0 from unw_step, then PAL calls unw_is_signal_frame with
c->validate==0 which in turn dereferences zeroed ip in access_mem.
libunwind spec says that retcode 0 from unw_step means very end of a
stack, so PAL should not expect any frames, signal or not. It should
convert cursor back to SEH representation and return with TRUE.