Use GCC 10 branch in build-many-glibcs.py.

This updates the default GCC version used in build-many-glibcs.py when
no version is specified explicitly.  I'm replacing my bot using GCC 8
with one using GCC 10 (leaving the GCC 9 and GCC mainline bots running
as at present).

scripts/build-many-glibcs.py

@@ -747,7 +747,7 @@ class Context(object):
     def checkout(self, versions):
         """Check out the desired component versions."""
         default_versions = {'binutils': 'vcs-2.34',
-                            'gcc': 'vcs-9',
+                            'gcc': 'vcs-10',
                             'glibc': 'vcs-mainline',
                             'gmp': '6.2.0',
                             'linux': '5.6',