gdbsupport: add observer_debug_printf, OBSERVER_SCOPED_DEBUG_ENTER_EXIT

gdbsupport: add observer_debug_printf, OBSERVER_SCOPED_DEBUG_ENTER_EXIT

Switch observer to use the "new" debug printf mechanism and sprinkle a
few debug prints.  Here's a small example of the output with "infrun"
and "observer" debug output enabled:

    [infrun] proceed: enter
      [observer] notify: start: observable target_resumed notify() called
        [observer] notify: start: calling observer mi-interp of observable target_resumed
        [observer] notify: end: calling observer mi-interp of observable target_resumed
        [observer] notify: start: calling observer py-inferior of observable target_resumed
        [observer] notify: end: calling observer py-inferior of observable target_resumed
      [observer] notify: end: observable target_resumed notify() called
      ...

gdbsupport/ChangeLog:

	* observable.h (observer_debug_printf,
	OBSERVER_SCOPED_DEBUG_START_END): New.
	(class observable) <notify, attach>: Use them.

Change-Id: If3ae4b6b65450ca3b7cae56698a87fc526688b86