Make build-many-glibcs.py re-exec itself if changed by checkout.

Updating build-many-glibcs.py may result in changes to the default
versions of components, or to the set of components (if e.g. Hurd
support is added and that requires a new component).

It's desirable for the checkout process to leave a source tree that is
ready to use.  If the checkout updated the script itself, that means
it needs to be rerun to cause any new versions or components
referenced by the new script version to be properly checked out.  This
patch makes the script check if it was modified by the checkout
process, and re-exec itself (with the same arguments) if so.

	* scripts/build-many-glibcs.py (Context.__init__): Save text of
	script being executed.
	(Context.get_script_text): New function.
	(Context.exec_self): Likewise.
	(Context.checkout): Re-exec script if changed by checkout process.