[lld][WebAssembly] Fix for string merging + negative addends
Don't include the relocation addend when calculating the
virtual address of a symbol. Instead just pass the symbol's
offset and add the addend afterwards.
Without this fix we hit the offset is outside the section
error in MergeInputSegment::getSegmentPiece.
This fixes a real world error we were are seeing in emscripten.
Differential Revision: https://reviews.llvm.org/D102271