Added rudimentary impl of DW_OP_push_object_address for VLA.

Added a field to hold the address of a variable length array
object to the baton struct in dwarfloc. With DW_OP_push_obj-
ect_address, a callback function is invoked which returns
the address of the object.

gdb/:

 * dwarf2expr.c (execute_stack_op): Added case for
   DW_OP_push_object_address opcode which returns the VLA
   object address.
 * dwarf2expr.h (get_object_addr): Added function pointer
   which returns the VLA objects' CORE_ADDR.
 * dwarf2loc.c (dwarf_expr_get_obj_addr): Implemented
   callback function for DW_OP_push_object_address and
   added function to virtual method table.
 * dwarf2loc.h (dwarf2_locexpr_baton): Added field to hold
   the address of our object.
 * dwarf2read.c (dwarf_fetch_die_loc_sect_off): Implemented
   value assignment to new field in dwarf2_loc_expr_baton.
   (dwarf2_read_obj_addr): Function implemented
   which returns an object address, if available.

Change-Id: Id6348eac0ed0f2337d941898185f8644bac603d4