Fix Solaris build

The recent commit 08036331 ("Per-inferior thread list, thread
ranges/iterators, down with ALL_THREADS, etc.") removed the
definitions of is_running/is_stopped/is_exited but missed updating a
couple uses of is_exited in Solaris-specific code.

Tested by Rainer Orth on amd64-pc-solaris2.11.

gdb/ChangeLog:
2018-11-26  Pedro Alves  <palves@redhat.com>

	* procfs.c (procfs_notice_thread): Replace uses of
	in_thread_list/is_exited with find_thread_ptid/THREAD_EXITED.
	* sol-thread.c (sol_thread_target::wait)
	(sol_update_thread_list_callback): Likewise.