[WebAssembly] Create synthetic __dso_handle symbol

This is used by __cxa_ataxit to determine the currently
executing DLL.  Once we fully support DLLs this will need
to be set to some address within the DLL.

The ELF linker added support for this symbol here:
https://reviews.llvm.org/D33856

Differential Revision: https://reviews.llvm.org/D42024

llvm-svn: 322606