Use std::forward_list for displaced_step_inferior_states

Use std::forward_list for displaced_step_inferior_states

Use std::forward_list instead of manually implemented list.  This
simplifies a bit the code, especially around removal.

Regtested on the buildbot.  There are some failures as always, but I
think they are unrelated.

gdb/ChangeLog:

	* infrun.c (displaced_step_inferior_states): Change type to
	std::forward_list.
	(get_displaced_stepping_state): Adjust.
	(displaced_step_in_progress_any_inferior): Adjust.
	(add_displaced_stepping_state): Adjust.
	(remove_displaced_stepping_state): Adjust.