powerpc/crash: save cpu register data in crash_smp_send_stop()

During kdump, two set of NMI IPIs are sent to secondary CPUs, if
'crash_kexec_post_notifiers' option is set. The first set of NMI IPIs
to stop the CPUs and the other set to collect register data. Instead,
capture register data for secondary CPUs while stopping them itself.
Also, fallback to smp_send_stop() in case the function gets called
without kdump configured.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220630064942.192283-1-hbathini@linux.ibm.com