[WebAssembly] Error on R_WASM_MEMORY_ADDR relocations against undefined data symbols.

For these types of relocations an absolute memory address is
required which is not possible for undefined data symbols.  For symbols
that can be undefined at link time (i.e. external data symbols in
shared libraries) a different type of relocation (i.e. via a GOT) will
be needed.

Differential Revision: https://reviews.llvm.org/D59337

llvm-svn: 356310