gdb, breakpoint: add a destructor to the watchpoint struct

gdb, breakpoint: add a destructor to the watchpoint struct

Make sure to unlink the related breakpoint when the watchpoint instance
is deleted.  This prevents having a wp-related breakpoint that is
linked to a NULL watchpoint (e.g.  the watchpoint instance is being
deleted when the 'watch' command fails).  With the below scenario,
having such a left out breakpoint will lead to a GDB hang, and this
is due to an infinite loop when deleting all inferior breakpoints.

Scenario:
	(gdb) set can-use-hw-watchpoints 0
	(gdb) awatch <SCOPE VAR>
	Can't set read/access watchpoint when hardware watchpoints are disabled.
	(gdb) rwatch <SCOPE VAR>
	Can't set read/access watchpoint when hardware watchpoints are disabled.
	(gdb) <continue the program until the end>
	>> HANG <<

Signed-off-by: Mohamed Bouhaouel <mohamed.bouhaouel@intel.com>
Reviewed-by: Bruno Larsen <blarsen@redhat.com>