Use Linux 5.0 in build-many-glibcs.py.

This patch makes build-many-glibcs.py use Linux 5.0 in place of 4.20
(now that the test change required to avoid false positives with ulong
in kernel headers has been committed).  This includes adjusting the
logic to compute a tarball URL to handle different major version
numbers (rather than changing the path to hardcode v5.x in place of
v4.x, as someone might still wish to check out a v4.x version).

Tested that build-many-glibcs.py successfully checks out Linux 5.0
sources after this patch.

	* scripts/build-many-glibcs.py (Context.checkout): Default Linux
	version to 5.0.
	(Context.checkout_tar): Handle variable major version for Linux
	kernel.