SMP - can boot even if some cpus fail to boot

- EBADCPU is returned is scheduler tries to run a process on a CPU
  that either does not exist or isn't booted

- this change was originally meant to deal with stupid cpuid
  instruction which provides totally useless information about
  hyper-threading and MPS which does not deal with ht at all. ACPI
  provides correct information. If ht is turned off it looks like some
  CPUs failed to boot.  Nevertheless this patch may be handy for
  testing/benchmarking in the future.

kernel/arch/i386/arch_smp.c

@@ -36,8 +36,8 @@ extern void * __trampoline_end;
 extern u32_t busclock[CONFIG_MAX_CPUS];
 extern int panicking;
 
-static int ap_cpu_ready;
-static int cpu_down;
+static int volatile ap_cpu_ready;
+static int volatile cpu_down;
 
 /* there can be at most 255 local APIC ids, each fits in 8 bits */
 PRIVATE unsigned char apicid2cpuid[255];
@@ -186,6 +186,11 @@ PUBLIC void smp_shutdown_aps(void)
 	for (cpu = 0; cpu < ncpus; cpu++) {
 		if (cpu == cpuid)
 			continue;
+		if (!cpu_test_flag(cpu, CPU_IS_READY)) {
+			printf("CPU %d didn't boot\n", cpu);
+			continue;
+		}
+
 		cpu_down = -1;
 		barrier();
 		apic_send_ipi(APIC_SMP_CPU_HALT_VECTOR, cpu, APIC_IPI_DEST);