tc-i960.c: add some casts when assigning literals to args[i]

tc-i960.c: add some casts when assigning literals to args[i]

parse_ldconst () takes a char ** as a in / out argument, and sometimes points
args[0] to a constant string.  Then in some cases after parse_ldconst ()
     returns md_assemble () twiddles the contents of arg[0].  So it seems like
     it would take some work to avoid these casts, and its not really clear
     that work is worth it.

gas/ChangeLog:

2016-03-31  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* config/tc-i960.c (parse_ldconst): Cast to char * when assigning to
	args[0].