I had this problem, and I simply modified the EFI partition, changing the boot entries to
/EFI/archiso/vmlinuz.efi instead of
/EFI/archiso/vmlinuz. I think the ISO is flawed for EFI systems (or at least the fallback options are on 17.9 from my testing).
Edit: So it is flawed in 17.10 as well, and here’s how to fix it:
Step 1. Mount your EFI partition.
Step 2. Navigate to /loader/entries, and find the entry you’re trying to use. Rinse and repeat for all of them if you plan on using all of them.
Step 3. Open up your desired entry and find add
.efi onto this line:
Step 4. Save, and repeat steps 2-3 for all of the entries you plan on using. Hopefully the @developers will end up fixing this sooner or later.