gdb: add program_space parameter to get_symbol_leading_char

Make the current_program_space references bubble up one level.  In this
case, I think it makes sense to use m_objfile's program space.

Change-Id: Ibecb89b5e8a0363328240f1675d0fb95ff99c99a
Reviewed-by: Keith Seitz <keiths@redhat.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>