[ELF] Move Symbol::needsTlsLd to config->needsTlsLd

to decrease sizeof(SymbolUnion) from 72 to 64 on ELF64 platforms.

Use a dummy `Undefined` to prevent null pointer dereference (though unused)
`*rel.sym` in InputSectionBase::relocateAlloc.

The relocation order may shuffle a bit, but otherwise there is no behavior
difference.