Don't write to inferior_ptid in nto-procfs.c

Don't write to inferior_ptid in nto-procfs.c

A best effort patch, which fixes some bit rot and removes some
inferior_ptid references -- this port clearly hasn't been built in a
long while.

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

	* nto-procfs.c (nto_procfs_target::update_thread_list): Avoid
	inferior_ptid.
	(nto_procfs_target::attach): Avoid inferior_ptid.  Switch to
	thread.
	(nto_procfs_target::detach): Avoid referencing
	inferior_ptid.  Use switch_to_no_thread instead of writing to
	inferior_ptid directly.
	(nto_procfs_target::mourn_inferior): Use switch_to_no_thread
	instead of writing to inferior_ptid directly.
	(nto_procfs_target::create_inferior): Avoid inferior_ptid.  Switch
	to thread.