[MI] Extending -list-thread-groups --available to show cores

[Vladimir Prus <vladimir@codesourcery.com>]

Current GDB has a MI command -list-thread-groups to nagivate
the hierarchy of the debugged thing. This command also the the
--available option that cause GDB to report the processes that
can be attached to, as opposed to processes currently being
debugged.

We were recently asked to slightly extend the returned information
to include the core where each thread runs. Such information is
of little use for typical Linux application, since threads are
migrated between cores. However, it's useful for both custom 
Linux applications that specifically pin threads to specific cores,
and for embedded systems. Therefore, I plan to add a new field
to the thread information that is output by 
-list-thread-groups --available, named 'core' that will give the
number of the core. E.g.

	-list-thread-groups
	^done,groups=[{id="17",type="process",pid="yyy",num_children="2",cores=[1,2]}]
	-list-thread-groups 17
	^done,threads=[{id="2",target-id="Thread 0xb7e14b90 (LWP 21257)",cores=[1]
	   frame={level="0",addr="0xffffe410",func="__kernel_vsyscall",args=[]},state="running"},

Related to that, it would be somewhat inefficient to issue -list-thread-groups
for every avaialable process to discover the full list of threads on specific
core, so it would be nice to pass several thread groups, like so:

	-list-thread-groups 17 18
	^done,groups=[{id="17", types="process", ..., 
                   threads=[{id="2",target-id="Thread 0xb7e14b90 (LWP 21257)",cores=[1]}]},
			      {id="18", types="process", ..., 

Finally, we were asked to make the *stopped notification to report the core on which
the stop has happened, as additional field.

While those changes seem relatively minor and in line with previous MI developments,
I wanted to pass them here. If there are no objections, I'll work in implementation
during coming weeks.

Thanks,
Volodya