[lld][WebAssembly] Handle weakly defined symbols in shared libraries.

In the case of weakly defined symbols in shared libraries we now
generate both an import and an export.  The dynamic linker can then
choose how a winner from among all the shared libraries that define a
given symbol.

Previously any direct usage of a weakly defined symbol would use the
DSO-local definition (For example, even through there would be single
address for a weakly defined function, each DSO could end up directly
calling its local version).

Fixes: https://github.com/emscripten-core/emscripten/issues/13773

Differential Revision: https://reviews.llvm.org/D108413