Yield while spinning on APs and avoid announcing all CPUs unless bootverbose

is set. These improve startup performance on massively multithreaded systems
with 8-way SMT and dozens to hundreds of CPUs.

MFC after:	3 weeks

sys/powerpc/powerpc/mp_machdep.c

@@ -78,7 +78,8 @@ machdep_ap_bootstrap(void)
 	__asm __volatile("msync; isync");
 
 	while (ap_letgo == 0)
-		;
+		__asm __volatile("or 27,27,27");
+	__asm __volatile("or 6,6,6");
 
 	/* Initialize DEC and TB, sync with the BSP values */
 	platform_smp_timebase_sync(ap_timebase, 1);
@@ -177,6 +178,9 @@ cpu_mp_announce(void)
 	struct pcpu *pc;
 	int i;
 
+	if (!bootverbose)
+		return;
+
 	CPU_FOREACH(i) {
 		pc = pcpu_find(i);
 		if (pc == NULL)