Previously lookupSymbol_PEi386 would call lookupSymbol while
holding linker_mutex. Fix this by rather
calling `lookupDependentSymbol`. This is safe
because lookupSymbol_PEi386 unconditionally holds linker_mutex.

Happily, this un-breaks `T12771`, `T13082_good`, and `T14611`, which
were previously marked as broken due to #18718.

Closes #19155.