Make yield a regular syscall taking an argument.

Allows "wait for interrupt"/"wait for event" type operation where VM code blocks until the host has something for it.

apps/sketch/sketch.c

@@ -25,7 +25,7 @@ void setup(void) {
 void main(void) {
     setup();
     while(loop()) {
-        yield();
+        yield(0);
     }
 }