[WebAssembly] Write out undefined symbol addresses as zero

The addresses of undefined symbols that make it into the final
executable (i.e. weak references to non-existent symbols) should
resolve to zero.

Also, make sure to not include function in the indirect function
table if they are not included in the output.

Differential Revision: https://reviews.llvm.org/D41839

llvm-svn: 322045