[binutils-gdb] Use std::forward_list for displaced_step_inferior_states

[sergiodj+buildbot at sergiodj dot net]

*** TEST RESULTS FOR COMMIT 39a36629f68e9796d950d9204012fe8272e2d0ef ***

Author: Simon Marchi <simon.marchi@ericsson.com>
Branch: master
Commit: 39a36629f68e9796d950d9204012fe8272e2d0ef

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.