powerpc: Don't savectx() twice in IPI_STOP handler

We already save context in stoppcbs[] array, so there's no need to also save it
in the PCB, it won't be used.

sys/powerpc/powerpc/mp_machdep.c

@@ -331,7 +331,6 @@ powerpc_ipi_handler(void *arg)
 			    __func__);
 			cpuid = PCPU_GET(cpuid);
 			savectx(&stoppcbs[cpuid]);
-			savectx(PCPU_GET(curpcb));
 			CPU_SET_ATOMIC(cpuid, &stopped_cpus);
 			while (!CPU_ISSET(cpuid, &started_cpus))
 				cpu_spinwait();