gem5: update to 4c8efdbef45d98109769cf675ee3411393e8ed06

The Linux boot was still broken after the previous gem5 update. Let's try
a new one...

Add the --no-ccache option to benchmark builds.

build-gem5

@@ -121,6 +121,9 @@ https://github.com/cirosantilli/linux-kernel-module-cheat-regression#gem5-unit-t
                 gold_linker_cmd = []
             else:
                 gold_linker_cmd = ['--gold-linker', LF]
+        kwargs = {}
+        if self.env['ccache']:
+            kwargs['extra_paths'] = [self.env['ccache_dir']]
         exit_status = self.sh.run_cmd(
             (
                 [
@@ -141,9 +144,9 @@ https://github.com/cirosantilli/linux-kernel-module-cheat-regression#gem5-unit-t
                 self.sh.add_newlines(self.env['extra_scons_args'])
             ),
             cwd=self.env['gem5_source_dir'],
-            extra_paths=[self.env['ccache_dir']],
             extra_env=extra_env,
             raise_on_failure = False,
+            **kwargs
         )
         return exit_status