Use GCC 8 in build-many-glibcs.py by default.

Now that GCC 8 has branched, this patch makes build-many-glibcs.py
default to using GCC 8 branch instead of GCC 7.  The effect should be
that all builds complete cleanly and the compilation parts of the
glibc testsuite complete cleanly except for on i686-gnu (with GCC 7
there were testsuite failures for some other configurations as well).

I've replaced my bot building using GCC 6 branch with one using GCC 8
branch.  (Of course glibc should continue building with GCC 6 - and
for that matter GCC 5 and 4.9, which are no longer maintained, are
supported versions as well - but building with GCC 6 will no longer be
included in my bot testing.)

	* scripts/build-many-glibcs.py (Context.checkout): Default GCC
	version to GCC 8 branch.