gdb, python: use gdbarch_iterate_over_objfiles_in_search_order

gdb, python: use gdbarch_iterate_over_objfiles_in_search_order

The implementation of gdb.lookup_objfile() iterates over all objfiles and
compares their name or build id to the user-provided search string.

This will cause problems when supporting linker namespaces as the first
objfile in any namespace will be found.  Instead, use
gdbarch_iterate_over_objfiles_in_search_order to only consider the
namespace of gdb.current_objfile() for the search, which defaults to the
initial namespace when gdb.current_objfile() is None.