mkarchiso: use -isohybrid-gpt-basdat instead of -appended_part_as_gpt for ISOs that will support BIOS booting

Some hardware, like Lenovo Thinkpad T420, will not BIOS boot if the disk has a valid GPT.
See https://bbs.archlinux.org/viewtopic.php?id=264096 .

Instead of a valid GPT, change to a valid MBR and invalid GPT similar to what was used before 729d16b4. That layout, despite having crazy partition tables, boots everywhere.
The difference is that -append_partition is still kept and specified before -isohybrid-gpt-basdat. Thus the appended partition will be listed as EFI system partition in MBR and as Microsoft basic partition in the invalid GPT.

Fixes #102.