[LLD] [COFF] Mark the symbol _tls_used as a GC root (#71336)

This is necessary if the object file containing _tls_used is built with
-fdata-sections; if this symbol is present, lld is going to include
references to it in the PE header, in the TLS_TABLE data directory.

Therefore, treat the symbol as a GC root, i.e. retain it (and whatever
other symbols the section chunk references) when doing doing GC
(/opt:ref), if the symbol is present.