s390: Clean up s390-linux-tdep.c
    
    After moving big parts of the code to the new s390-tdep.c file
    s390-linux-tdep.c now contains many includes it doesn't need anymore.
    Furthermore, there are some functions lacking a description.
    
    Fix both and order the remaining includes alphabetically.
    
    gdb/ChangeLog
    	* s390-linux-tdep.c: Remove unneeded includes and order them
    	alphabetically.  Add comments to functions without description.