extract_objects: skip headers when building custom_target command line

As seen in the testcase, passing objects to custom_target does not work
if headers are passed extract_objects(), or if extract_all_objects() is used
and the sources include any header files.  To fix this, use the code that
already exists for unity build to filter out the nonexistent ".h.o" files.
This already gives for free the handling of genlist, which was mentioned
in a TODO comment.

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