build-many-glibcs.py: Fix s390x-linux-gnu.

The recent commit 638d437dbf
"Deprecate s390-linux-gnu (31bit)"
leads to:
FAIL: compilers-s390x-linux-gnu gcc build
when it tries to build 31bit libgcc.

The build is fixed by explicitely disabling multilib.

scripts/build-many-glibcs.py

@@ -439,6 +439,7 @@ class Context(object):
                                  '--disable-multilib'])
         self.add_config(arch='s390x',
                         os_name='linux-gnu',
+                        gcc_cfg=['--disable-multilib'],
                         extra_glibcs=[{'variant': 'O3',
                                        'cflags': '-O3'},
                                       {'variant': 'zEC12',