zasm: invert emulator io_write() handling

This facilitates debugging. To know the value of `A` at any point,
you can do `out (0), a`. The number of bytes in the output will be the
value of `A`.

apps/zasm/emul/glue.asm

@@ -18,7 +18,8 @@ init:
 	call	USER_CODE
 	; signal the emulator we're done
 	; BC contains the number of written bytes
-	ld	a, b
+	ld	a, c
+	ld	c, b
 	out	(c), a
 	halt
 

apps/zasm/emul/zasm.c

@@ -32,7 +32,7 @@ static uint8_t io_read(int unused, uint16_t addr)
 
 static void io_write(int unused, uint16_t addr, uint8_t val)
 {
-    written = (val << 8) + (addr & 0xff);
+    written = ((addr & 0xff) << 8) + (val & 0xff);
     running = 0;
 }