build-many-glibcs.py: Use make -O for more consistent log output

The -O option avoids interleaving (e.g.) compiler error messages
with other build messages, and makes attribution of messages to
commands easier.

According to my tests, the impact on build time is within the noise.

scripts/build-many-glibcs.py

@@ -670,7 +670,7 @@ class Context(object):
 
     def do_build(self):
         """Do the actual build."""
-        cmd = ['make', '-j%d' % self.parallelism]
+        cmd = ['make', '-O', '-j%d' % self.parallelism]
         subprocess.run(cmd, cwd=self.builddir, check=True)
 
     def build_host_libraries(self):