Add parametrized version of s390_inferior_tid
    
    In the next patches, we'll want to use this function with a ptid other
    than inferior_ptid, so I add it with a separate patch to keep things
    clean.
    
    gdb/ChangeLog:
    
    	* s390-linux-nat.c (s390_inferior_tid): Add overload with ptid
    	parameter.