gdb: make some breakpoint methods use `this`

gdb: make some breakpoint methods use `this`

Some implementations of breakpoint::check_status and
breakpoint::print_it do this:

    struct breakpoint *b = bs->breakpoint_at;

bs->breakpoint_at is always the same as `this` (we can get convinced by
looking at the call sites of check_status and print_it), so it would
just be clearer to access fields through `this` instead.

Change-Id: Ic542a64fcd88e31ae2aad6feff1da278c7086891
Reviewed-By: Alexandra Petlanova Hajkova <ahajkova@redhat.com>
Reviewed-By: Andrew Burgess <aburgess@redhat.com>