[WebAssembly] Model weakly defined symbols as wasm exports

Previously these were being included as both imports and
exports, with the import being satisfied by the export
(or some strong symbol) at runtime.  However proved
unnecessary and actually complicated linking as it meant
there was not a 1-to-1 mapping between a wasm function
/global index and a linker symbol.

Differential Revision: https://reviews.llvm.org/D38246

llvm-svn: 314245