Don't write to inferior_ptid in remote-sim.c

gdb/ChangeLog:
2020-06-18  Pedro Alves  <palves@redhat.com>

	* remote-sim.c (gdbsim_target::create_inferior): Switch to thread
	after creating it, instead of writing to inferior_ptid.
	(gdbsim_target_open): Use switch_to_no_thread instead of writing
	to inferior_ptid directly.
	(gdbsim_target::wait): Don't write to inferior_ptid.