[gdb] Make addrmap_mutable::set_empty return bool

Function addrmap_mutable::set_empty has the follow behavior (shortened
comment):
...
/* In the mutable address map MAP, associate the addresses from START
   to END_INCLUSIVE that are currently associated with NULL with OBJ
   instead.  Addresses mapped to an object other than NULL are left
   unchanged.  */
  void set_empty (CORE_ADDR start, CORE_ADDR end_inclusive,
		  void *obj);
...

Change the return type to bool, and return true if the full range
[START, END_INCLUSIVE] is mapped to OBJ.

Tested on x86_64-linux.

Approved-By: Simon Marchi <simon.marchi@efficios.com>