powerpc/64s: Fix build failures with CONFIG_NMI_IPI=n

I broke the build when CONFIG_NMI_IPI=n with my recent commit to add
arch_trigger_cpumask_backtrace(), eg:

  stacktrace.c:(.text+0x1b0): undefined reference to `.smp_send_safe_nmi_ipi'

We should rework the CONFIG symbols here in future to avoid these
double barrelled ifdefs but for now they fix the build.

Fixes: 5cc05910 ("powerpc/64s: Wire up arch_trigger_cpumask_backtrace()")
Reported-by: Christophe LEROY <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>