[WebAssembly] Add explicit symbol table
This change modified lld to in response the llvm change which
moved to a more explicit symbol table in the object format.
Based on patches by Nicholas Wilson:
The primary difference that we see in the test output is that
for relocatable (-r) output we now have symbol table which
replaces exports/imports and globals.
Differential Revision: https://reviews.llvm.org/D43264