libdw: Skip imported compiler_units in libdw_visit_scopes walking DIE tree

libdw: Skip imported compiler_units in libdw_visit_scopes walking DIE tree

Some gcc -flto versions imported other top-level compile units,
skip those. Otherwise we'll visit various DIE trees multiple times.

Note in the testcase that with newer GCC versions function foo is
fully inlined and does appear only once (as declared, but not as
separate subprogram).

Signed-off-by: Mark Wielaard <mark@klomp.org>