Tie XIF to 0 in x_heep_system if X_EXT=0

[davidmallasen]

This would remove the need of creating a dummy xif and connecting it to the ports of x_heep_system as done in the tb/testharness.sv with:

// eXtension Interface
  if_xif #() ext_if ();

...
      .xif_compressed_if(ext_if),
      .xif_issue_if(ext_if),
      .xif_commit_if(ext_if),
      .xif_mem_if(ext_if),
      .xif_mem_result_if(ext_if),
      .xif_result_if(ext_if),
...

[StMiky]

To make X-HEEP compatible with older projects, we may use templates to generate the CORE-V-XIF ports only when the selected CPU is cv32e40x. Pros:

• Do not break compatibility with older projects.
• Do not force users that don't need the XIF to instantiate it in their top-level module.

Cons:

• Additional templates required (including for the testbench)
• Systems that can be configured to use both XIF and non-XIF CPUs must handle two variants of X-HEEP top-level interface (e.g., using ifdef MACRO guards).