wake application processors to long mode

INIT-SIPI-SIPI plus a self-relocating real-mode trampoline.

src/kernel/scheduler.zig

@@ -102,6 +102,25 @@ fn idle() void {
     while (true) asm volatile ("hlt");
 }
 
+/// Reserve and initialise the per-CPU slot for an application processor at dense
+/// `index` (1-based; 0 is the BSP) with Local APIC id `apic_id`, and return a
+/// pointer the arch bring-up hands to the core (it publishes it in its GS base).
+/// Called on the BSP before waking each AP; the AP marks itself `online`.
+pub fn prepareSecondary(index: usize, apic_id: u32) *PerCpu {
+    const pc = &cpus[index];
+    pc.* = .{ .index = @intCast(index), .apic_id = apic_id, .online = false };
+    return pc;
+}
+
+/// Number of cores that have finished bring-up (the BSP plus every online AP).
+pub fn onlineCount() usize {
+    var n: usize = 0;
+    for (&cpus) |*pc| {
+        if (pc.online) n += 1;
+    }
+    return n;
+}
+
 fn enqueue(t: *Task) void {
     t.next = null;
     const p: usize = t.priority;