[arch][x86][smp] few misc tweaks

- Skip cpus in the MADT table that are not enabled
- Bump count to 16 cpus
- Move the spurious interrupt vector to 0xff since it needs to end in
  0xf on <=P6.

arch/x86/lapic.c

@@ -84,9 +84,10 @@ enum lapic_regs {
 
 enum lapic_interrupts {
     LAPIC_INT_TIMER = 0xf8,
-    LAPIC_INT_SPURIOUS,
     LAPIC_INT_GENERIC,
     LAPIC_INT_RESCHEDULE,
+
+    LAPIC_INT_SPURIOUS = 0xff, // Bits 0-3 must be 1 for P6 and below compatibility
 };
 
 enum lapic_timer_mode {

arch/x86/rules.mk

@@ -52,7 +52,7 @@ GLOBAL_DEFINES += \
 	ARCH_HAS_MMU=1
 
 ifeq ($(WITH_SMP),1)
-SMP_MAX_CPUS ?= 8
+SMP_MAX_CPUS ?= 16
 GLOBAL_DEFINES += \
     WITH_SMP=1 \
     SMP_MAX_CPUS=$(SMP_MAX_CPUS)

platform/pc/mp.c

@@ -108,6 +108,10 @@ static void local_apic_callback(const void *_entry, size_t entry_len, void *cook
     const struct acpi_madt_local_apic_entry *entry = _entry;
     struct detected_cpus *cpus = cookie;
 
+    if ((entry->flags & ACPI_MADT_FLAG_ENABLED) == 0) {
+        return;
+    }
+
     // TODO: read the current APIC id and skip it, instead of assuming 0 is the boot cpu
     // read BSP from X86_IA32_APIC_BASE_MSR bit 8?
     if (entry->apic_id == 0) {