Use GCC 12 branch in build-many-glibcs.py

This patch makes build-many-glibcs.py use GCC 12 branch by default.

Tested with build-many-glibcs.py (host-libraries, compilers and glibcs
builds).

scripts/build-many-glibcs.py

@@ -786,7 +786,7 @@ class Context(object):
     def checkout(self, versions):
         """Check out the desired component versions."""
         default_versions = {'binutils': 'vcs-2.38',
-                            'gcc': 'vcs-11',
+                            'gcc': 'vcs-12',
                             'glibc': 'vcs-mainline',
                             'gmp': '6.2.1',
                             'linux': '5.18',