[MC] Set sh_link to 0 if the associated symbol is undefined

Part of https://bugs.llvm.org/show_bug.cgi?id=41734

LTO can drop externally available definitions. Such AssociatedSymbol is
not associated with a symbol. ELFWriter::writeSection() will assert.

Allow a SHF_LINK_ORDER section to have sh_link=0.

We need to give sh_link a syntax, a literal zero in the linked-to symbol
position, e.g. `.section name,"ao",@progbits,0`

Reviewed By: pcc

Differential Revision: https://reviews.llvm.org/D72899