gdb: make `program_space::free_all_objfiles` use `this`

gdb: make `program_space::free_all_objfiles` use `this`

Use `this` instead of `current_program_space`.  Presumably, the method
wants to check the solibs of "this" program space, not the current
global program space (although they are likely always the same at the
moment).

Change-Id: Iaf0534f36bfd47c04c53ed0657da332bdb8fb906
Approved-By: Tom Tromey <tom@tromey.com>
Reviewed-By: Thiago Jung Bauermann <thiago.bauermann@linaro.org>