Default build-many-glibcs.py to GCC 7 branch.

Now there is a GCC 7 release branch, this patch makes
build-many-glibcs.py default to using it instead of GCC 6.

Relative to GCC 6, this fixes the MicroBlaze build but introduces ICEs
building glibc testcases for SH (GCC PRs 78459, 78460; the latter is
an out-of-memory issue so you may want to set memory limits when
running build-many-glibcs.py).

	* scripts/build-many-glibcs.py (Context.checkout): Default gcc
	version to 7 branch.