Make cmd_descs_generate automatically set LAST flag when the arg is last

[ret2libc]

Is your feature request related to a problem? Please describe.
While converting commands, I often find myself having to manually set RZ_CMD_ARG_FLAG_LAST flag in the last argument of a command.

Describe the solution you'd like
This issue is to investigate if it's possible to automatically set the LAST flag to the last argument in the list, unless the argument already has flags specified. This way, it is less error prone and easier for the devs as well.