[icache] Define some fake DPI functions to simplify linking

This is triggered by the fact that if the ICache parameter is false
then we don't instantiate the ibex_icache module. For verilator
simulations, the module is then discarded entirely, which means that
its two DPI functions are not defined. That's unfortunate because
we're also compiling the code in scrambled_ecc32_mem_area.cc, which
expects the functions to be defined.

The obvious solution (don't include scrambled_ecc32_mem_area.cc if you
don't have an icache) isn't easy to do, because FuseSoc doesn't
currently allow us to use parameters to configure its dependency
tree (see fusesoc issue 438 for a discussion).

The super-clever solution that I came up with before(!) was to declare
these symbols as weak in the C++ code. That way, we can do a runtime
check to make sure that no-one is silly enough to call them without an
icache, but everything will still build properly either way.

Unfortunately, that doesn't work well with xcelium simulations.
Xcelium turns out to compile all the C++ code into one .so library and
generate functions for exported DPI functions in another. These two
solibs then get loaded at runtime with dlopen(). But this doesn't work
with weak symbols: in fact, it seems you end up with the C++ version
every time. Boo!

So let's be stupider about it and define (bogus) versions of the DPI
functions in this case. Fortunately, both of them are designed to
return zero on failure so we can just return zero and needn't worry
too much.

The idea is that when this lands, we can revert the OpenTitan change
that switched the C++ code to using weak symbols and Xcelium
simulations will start working.

rtl/ibex_if_stage.sv

@@ -301,6 +301,23 @@ module ibex_if_stage import ibex_pkg::*; #(
     assign ic_data_write_o       = 'b0;
     assign ic_data_addr_o        = 'b0;
     assign ic_data_wdata_o       = 'b0;
+
+`ifndef SYNTHESIS
+    // If we don't instantiate an icache and this is a simulation then we have a problem because the
+    // simulator might discard the icache module entirely, including some DPI exports that it
+    // implies. This then causes problems for linking against C++ testbench code that expected them.
+    // As a slightly ugly hack, let's define the DPI functions here (the real versions are defined
+    // in prim_util_get_scramble_params.svh)
+    export "DPI-C" function simutil_get_scramble_key;
+    export "DPI-C" function simutil_get_scramble_nonce;
+    function automatic int simutil_get_scramble_key(output bit [127:0] val);
+      return 0;
+    endfunction
+    function automatic int simutil_get_scramble_nonce(output bit [319:0] nonce);
+      return 0;
+    endfunction
+`endif
+
   end
 
   assign unused_fetch_addr_n0 = fetch_addr_n[0];