[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