Add thread-exit annotation.

gdb/ChangeLog
2019-04-26  Amos Bird  <amosbird@gmail.com>

	* annotate.c (annotate_thread_exited): Add "thread-exited"
	annotation.

gdb/doc/ChangeLog
2019-06-06  Amos Bird  <amosbird@gmail.com>

	* annotate.texinfo (Multi-threaded Apps): Add entry for thread-exited
	annotation.

gdb/testsuite/ChangeLog
2019-06-06  Amos Bird  <amosbird@gmail.com>

	* gdb.base/annota1.exp (thread_switch): Add test for
	thread-exited annotation.