s390: Move tdesc validation to separate function
    
    Simplify s390_gdbarch_init by moving the target description validation to
    a separate function.
    
    gdb/ChangeLog:
    
    	* s390-linux-tdep.c (s390_tdesc_valid): New function.
    	(s390_validate_reg_range): New macro.
    	(s390_gdbarch_init): Adjust.