[ESI] Set '--output-split' for verilator compilation (llvm#8978)

* [ESI] Set '--output-split' for verilator compilation

... which also enables parallel compilation and better ccache support.

E.g. an internal simple test:
- without split: 8.2s compile time
- with split: 1.3s compile time

* correct flags

---------

Co-authored-by: John Demme <[email protected]>

Author: mortbopet

lib/Dialect/ESI/runtime/cosim_dpi_server/esi-cosim.py

@@ -264,8 +264,11 @@ def compile_commands(self) -> List[List[str]]:
         "-Wno-TIMESCALEMOD",
         "-Wno-fatal",
         "-sv",
-        "--build",
         "--exe",
+        "--build",
+        "-j",
+        "0",
+        "--output-split",
         "--assert",
         str(Verilator.DefaultDriver),
     ]