computing checksum for romsize 0x4000

wlalink now seems to place correct checksum (0x3FFA-0x3FFB) and
regioncode/romsize byte (0x3FFF) for 16KB ROMs.

Unfortunately, I still can't make .SMSTAG work, so I'm using the following workaround:

```
.SMSTAG  ; it's still not working properly, so we'll place the TMR signature 'by hand'
.org $3FF0
.section "TMR SEGA" force
  .db $54,$4D,$52,$20,$53,$45,$47,$41
.ends
```
how can I fix pass_2.c? It doesn't know the romsize... :|