add a big kernel lock for SMP

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

src/kernel/arch/x86_64/isr.s

@@ -63,9 +63,14 @@ switch_context:
     ret                     # return into the new task's saved instruction pointer
 
 # task_trampoline: the first thing a freshly-spawned task runs. init_task_stack
-# leaves its entry function in r15. New tasks start with interrupts enabled.
+# leaves its entry function in r15. A fresh task is switched to with the big kernel
+# lock held (the hand-off rule in sync.zig) but has no enter/leave frame of its own,
+# so it releases the lock here before running its body. r15 survives the call (it's
+# callee-saved). New tasks then start with interrupts enabled.
+.extern releaseForFreshTask
 .global task_trampoline
 task_trampoline:
+    call releaseForFreshTask # drop the kernel lock we inherited across the switch
     sti
     call *%r15              # call the task entry (fn() void)
 1:  hlt                     # if the entry returns, idle (still preemptible)