Just to clarify the comment from Marcin, the backup server doesn't have to be the same version as the ASE server. So it's possible to run combination of ASE 12.5.x and backupserver 15.7
Note that ASE 12.5 was only certified on RHEL 5.5, so even with ASE you might run into some issues on RHEL 7
If you really need to run ASE 12.5.1, maybe an idea to use RHEL 5.5 instead of RHEL 7?