[kernel] Save ring3 rflags in cpu_data, not just stack

So that kernel code can modify user rflags, save it in the CPU state
data, and save that off to the TCB when switching tasks.

src/kernel/objects/thread.h

@@ -15,6 +15,7 @@ struct TCB
     uintptr_t rsp;
     uintptr_t rsp0;
     uintptr_t rsp3;
+    uintptr_t rflags3;
     uintptr_t pml4;
 
     uint8_t priority;