gem5: update to bcf041f257623e5c9e77d35b7531bae59edc0423

This notably allow dynamic linking to work! Move entire README to it.

Also:

- explain what Dhrystone does and run it on gem5 to get DMIPS
- create getprops
- ./test-executables don't test files that start with "tmp."

run

@@ -498,6 +498,17 @@ Extra options to append at the end of the emulator command line.
                 ])
                 if self.env['userland_args'] is not None:
                     cmd.extend(['--options', self.env['userland_args'], LF])
+                if not self.env['static']:
+                    for path in self.env['userland_library_redirects']:
+                        cmd.extend([
+                            '--redirects',
+                            '{}={}'.format(
+                                os.sep + path,
+                                os.path.join(self.env['userland_library_dir'], path)
+                            ),
+                            LF
+                        ])
+                    cmd.extend(['--interp-dir', self.env['userland_library_dir'], LF])
             else:
                 if self.env['gem5_script'] == 'fs':
                     if self.env['gem5_restore'] is not None: