bfd: don't produce corrupt COFF symbol table due to long ELF file name symbols

bfd: don't produce corrupt COFF symbol table due to long ELF file name symbols

The re-writing logic in _bfd_coff_final_link() overwrote the ".file"
part of the symbol table entry, due to not coping with the auxiliary
entry generated in all cases.

Note that while I would have wanted to add a test case,
(a) I didn't spot any one testing the base functionality here, and
(b) I wasn't able to figure out proper conditionals to use in e.g.
    ld-elf/elf.exp to check for the necessary PE/PE+ support (which
    varies by target).