[WebAssembly][lld] Preassign table number 0 to indirect function table for MVP inputs

MVP object files may import at most one table, and if they do, it must
be assigned table number zero in the output, as the references to that
table are not relocatable.  Ensure that this is the case, even if some
inputs define other tables.

Differential Revision: https://reviews.llvm.org/D96001