Build failure in sim/rx/gdb-if.c on windows

Build failure in sim/rx/gdb-if.c on windows

This should fix a build failure reported on x86_64-mingw32 by Daniel
Calcoen due to conflicting declarations of "open".  This patch just
renames the static global in sim/rx/gdb-if.c into "rx_sim_is_open".

sim/rx/ChangeLog:

	* gdb-if.c (open): Rename to...
	(rx_sim_is_open): This. Replace all uses of "open" by uses of
	"rx_sim_is_open" throughout.

Tested by rebuilding on x86_64-linux.