Allow segments to auto-expand LoROM to size

[xragey]

Asar has the (in my opinion) undesirable behaviour where it does not auto "expand" a new rom being built from scratch while in lorom mode. (This works fine for hirom mode.) After filling the first $8000 bytes, further segments no longer work because the ROM has "no more freespace". This can be worked around with an org command (e.g. org $81FFFF : db $FF) but this requires beforehand knowledge of the size of the ROM to be built, which is not ideal.