gdb: add inferior_execd observable

gdb: add inferior_execd observable

I want to add another action (clearing displaced stepping state) that
happens when an inferior execs.  I think it would be cleaner to have an
observer for this event, rather than have infrun know about each other
sub-component.

Replace the calls to solib_create_inferior_hook and
jit_inferior_created_hook in follow_exec by observers.

gdb/ChangeLog:

	* observable.h (inferior_execd): Declare new observable.
	* observable.c (inferior_execd): Declare new observable.
	* infrun.c (follow_exec): Notify inferior_execd observer.
	* jit.c (jit_inferior_created_hook): Make static.
	(_initialize_jit): Register inferior_execd observer.
	* jit.h (jit_inferior_created_hook): Remove declaration.
	* solib.c (_initialize_solib): Register inferior_execd observer.

Change-Id: I000cce00094e23baa67df693d912646b6ae38e44