sim/rx: fix an issue where we try to modify a const string

sim/rx: fix an issue where we try to modify a const string

While experimenting with switching on warnings for the rx simulator I
discovered this bug.  In sim_do_command we get passed a 'const char *'
argument.  We create a copy of this string to work with locally, but
then while processing this we accidentally switch back to reference
the original string.

sim/rx/ChangeLog:

	* gdb-if.c (sim_do_command): Work with a copy of the command.