add a big kernel lock for SMP

Guards the scheduler and IPC; held across the context switch.

src/kernel/arch/x86_64/cpu.zig

@@ -282,3 +282,11 @@ pub fn readCr2() u64 {
 pub fn halt() noreturn {
     while (true) asm volatile ("hlt");
 }
+
+/// Spin-wait hint (`pause`). Emitted in the body of a spinlock's busy-wait: it
+/// relaxes the core while it polls a contended lock — yielding pipeline resources
+/// to a hyperthread sibling and easing the cache-coherency traffic on the lock
+/// line. Purely a performance/power hint; correct to omit, but kinder on the bus.
+pub fn cpuRelax() void {
+    asm volatile ("pause");
+}