PR 4437

    PR 4437
        * ldfile.c: (ldfile_open_file): Do not stop link upon encountering
        a missing file or library.  Instead mark the entry as missing and
        set the global flag to indicate that missing files were
        encountered.
        * ldlang.c (missing_files): New exported variable.
        (load_symbols): Skip loading if the file is missing.
        (open_input_bfds): Terminate link if any input files were
        missing.
        * ldlang.h (struct lang_input_statement_struct): Add missing_file
        field.
        Add export of missing_file variable.