Fix debugging with GDB when a breakpoint has multiple locations

Fix debugging with GDB when a breakpoint has multiple locations

* lisp/progmodes/gdb-mi.el (gdb-breakpoints--add-breakpoint-row):
New function, extracted from 'gdb-breakpoints-list-handler-custom'.
Don't print "in <unknown>" for header-rows of breakpoints with
multiple locations that don't have a function name attached.
(gdb-breakpoints-list-handler-custom): Add to the breakpoint table
also any locations in multiple-location breakpoints, which are
supported since GDB 6.8.