Add new threads to runnable threads list

More safely handle nested interrupts.
Switch to newly created thread immediately.

src/hulk/cpu.d

@@ -254,8 +254,7 @@ void cpuid1(uint * ebx, uint * ecx, uint * edx)
     __asm("cpuid", "=*{ebx},=*{ecx},=*{edx},{eax}", ebx, ecx, edx, 1u);
 }
 
-void swint(int swint_id)()
+void swint(ubyte swint_id)()
 {
-    static assert((0 <= swint_id) && (swint_id <= 255));
-    mixin(`__asm("int $$`, swint_id, `", "");`);
+    mixin(`__asm("int $$`, cast(int)cast(uint)swint_id, `", "");`);
 }