runtime_loader: keep symbol DSO in SymbolLookupCache

While resolving TLS related relocations it is necessary to know the DSO
that defines the symbol. Without proper support in caching that information
is available only when the symbol is resolved first time. That works well
for TLS since TLS_DTPMOD is guaranteed to be before TLS_DTPOFF relocation.
This patch makes the newly introduced parts of the interface work in a
general case.