Sybase ASE Auditing on sp_addlogin sp_droplogin sp_modifylogin
After trying hard on this without finding any workable solution, raised a case with Sybase support and they have provided solution.

Below are the commands that can be used to satisfy the mentioned requirement:

--Audit any change at procedure level for login/user/alias

use sybsystemprocs 
go 
sp_audit "exec_procedure", "all", "sybsystemprocs.dbo.sp_addlogin", "on" 
go 
sp_audit "exec_procedure", "all", "sybsystemprocs.dbo.sp_droplogin", "on" 
go 
sp_audit "exec_procedure", "all", "sybsystemprocs.dbo.sp_modifylogin", "on" 
go 
sp_audit "exec_procedure", "all", "sybsystemprocs.dbo.sp_adduser", "on" 
go 
sp_audit "exec_procedure", "all", "sybsystemprocs.dbo.sp_dropuser", "on" 
go

sp_audit "exec_procedure", "all", "sybsystemprocs.dbo.sp_addalias", "on" 
go

sp_audit "exec_procedure", "all", "sybsystemprocs.dbo.sp_dropalias", "on" 
go


--Audit any change at table level for login/user/alias 

sp_audit "delete", "all", "master.dbo.syslogins", "on"
go
sp_audit "insert", "all", "master.dbo.syslogins", "on"
go
sp_audit "update", "all", "master.dbo.syslogins", "on"
go

sp_audit "delete", "all", "master.dbo.sysusers", "on"
go
sp_audit "insert", "all", "master.dbo.sysusers", "on"
go
sp_audit "update", "all", "master.dbo.sysusers", "on"
go

sp_audit "delete", "all", "master.dbo.sysalternates", "on"
go
sp_audit "insert", "all", "master.dbo.sysalternates", "on"
go
sp_audit "update", "all", "master.dbo.sysalternates", "on"
go


sp_audit "delete", "all", "master.dbo.syssrvroles", "on"
go
sp_audit "insert", "all", "master.dbo.syssrvroles", "on"
go
sp_audit "update", "all", "master.dbo.syssrvroles", "on"
go


sp_audit "delete", "all", "master.dbo.sysloginroles", "on"
go
sp_audit "insert", "all", "master.dbo.sysloginroles", "on"
go
sp_audit "update", "all", "master.dbo.sysloginroles", "on"
go


I am sure this would help to anyone who wants to enable auditing at Sybase ASE level and is not sure how to achieve the same.