[build system] disable fortify hardening for verilated.nix

Also remove `--output-split`. It's no longer nedded.

nix/t1/verilated.nix

@@ -23,7 +23,6 @@ stdenv.mkDerivation {
       ${lib.optionalString enable-trace "--trace-fst"} \
       --timing \
       --threads 8 \
-      --output-split 2048 \
       -O3 \
       --cc TestBench
 
@@ -53,4 +52,9 @@ stdenv.mkDerivation {
 
     runHook postInstall
   '';
+
+  # nix fortify hardening add `-O2` gcc flag,
+  # we'd like verilator to controll optimization flags, so disable it.
+  # `-O2` will make gcc build time in verilating extremely long
+  hardeningDisable = [ "fortify" ];
 }