gdb: add breakpoint::first_loc methods

gdb: add breakpoint::first_loc methods

Add convenience first_loc methods to struct breakpoint (const and
non-const overloads).  A subsequent patch changes the list of locations
to be an intrusive_list and makes the actual list private, so these
spots would need to change from:

    b->loc

to something ugly like:

    *b->locations ().begin ()

That would make the code much heavier and not readable.  There is a
surprisingly big number of places that access the first location of
breakpoints.  Whether this is correct, or these spots fail to consider
the possibility of multi-location breakpoints, I don't know.  But
anyhow, I think that using this instead:

 b->first_loc ()

conveys the intention better than the other two forms.

Change-Id: Ibbefe3e4ca6cdfe570351fe7e2725f2ce11d1e95
Reviewed-By: Andrew Burgess <aburgess@redhat.com>