gdb: add program_space parameter to emit_clear_objfiles_event

Add program_space space parameters to emit_clear_objfiles_event and
create_clear_objfiles_event_object, making the reference to
current_program_space bubble up a bit.

Change-Id: I5fde2071712781e5d45971fa0ab34d85d3a49a71
Approved-By: Tom Tromey <tom@tromey.com>