gdbsupport: allow to specify dependencies between observers

gdbsupport: allow to specify dependencies between observers

Previously, the observers attached to an observable were always notified
in the order in which they had been attached.  That order is not easily
controlled, because observers are typically attached in _initialize_*
functions, which are called in an undefined order.

However, an observer may require that another observer attached only
later is called before itself is.

Therefore, extend the 'observable' class to allow explicitly specifying
dependencies when attaching observers, by adding the possibility to
specify tokens for observers that it depends on.

To make sure dependencies are notified before observers depending on
them, the vector holding the observers is sorted in a way that
dependencies come before observers depending on them.  The current
implementation for sorting uses the depth-first search algorithm for
topological sorting as described at [1].

Extend the observable unit tests to cover this case as well.  Check that
this works for a few different orders in which the observers are
attached.

This newly introduced mechanism to explicitly specify dependencies will
be used in a follow-up commit.

[1] https://en.wikipedia.org/wiki/Topological_sorting#Depth-first_search

Tested on x86_64-linux (Debian testing).

gdb/ChangeLog:

	* unittests/observable-selftests.c (dependency_test_counters):
	New.
	(observer_token0, observer_token1, observer_token2,
	observer_token3, observer_token4, observer_token5): New.
	(struct dependency_observer_data): New struct.
	(observer_dependency_test_callback): New function.
	(test_observers): New.
	(run_dependency_test): New function.
	(test_dependency): New.
	(_initialize_observer_selftest): Register dependency test.

gdbsupport/ChangeLog:

	* observable.h (class observable): Extend to allow specifying
	dependencies between observers, keep vector holding observers
	sorted so that dependencies are notified before observers
	depending on them.

Change-Id: I5399def1eeb69ca99e28c9f1fdf321d78b530bdb