WASM_x64: create object files

[certik]

We can create object files using the LLVM backend, we should also add this capability to the WASM_x64 backend, which currently can only create executables, but not object files. We should do this after we learn how to create sections in ELF.

The object files are useful if we want to use LCompilers to emit some code that would then be linked with some external application written in C, C++ or Fortran. We would simply emit appropriate object files and then link everything together with another application with an external linker.