elf: Never use the file ID of the main executable [BZ #24900]

If the loader is invoked explicitly and loads the main executable,
it stores the file ID of the main executable in l_file_id.  This
information is not available if the main excutable is loaded by the
kernel, so this is another case where the two cases differ.

This enhances commit 23d2e5fa
("elf: Self-dlopen failure with explict loader invocation
[BZ #24900]").

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Gabriel F. T. Gomes <gabrielftg@linux.ibm.com>