sim: constify dup_arg_p

sim: constify dup_arg_p

The dup_arg_p function is only given const strings, and it maintains a
local table of those const strings.  So constify the whole thing to fix
the GCC warnings:

common/sim-options.c: In function 'sim_parse_args':
common/sim-options.c:559: warning: passing argument 1 of 'dup_arg_p'
	discards qualifiers from pointer target type
common/sim-options.c: In function 'print_help':
common/sim-options.c:675: warning: passing argument 1 of 'dup_arg_p'
	discards qualifiers from pointer target type

Signed-off-by: Mike Frysinger <vapier@gentoo.org>