Add build-many-glibcs.py support for Hurd.

Add build-many-glibcs.py support for Hurd.

This patch adds build-many-glibcs.py support for GNU Hurd.  Builds of
the i686-gnu configuration will fail until sufficient support is
merged to master, so completing build-many-glibcs.py coverage of all
glibc ABIs and making results accurately reflect the broken state of
builds for Hurd.

	* scripts/build-many-glibcs.py (Context.add_all_configs): Add
	i686-gnu configurations.
	(Context.run_builds): Include mig, gnumach and hurd in components
	considered.
	(Context.checkout): Add mig, gnumach and hurd to components.
	(Context.checkout_tar): Add URL mappings for mig, gnumach and
	hurd.
	(Context.bot_cycle): Check for changes to mig, gnumach and hurd.
	(Config.build): Install gnumach headers, build mig and install
	hurd headers for 'gnu' OS.
	(Config.install_gnumach_headers): New function.
	(Config.install_hurd_headers): Likewise.
	(Glibc.build_glibc): Do not use /usr for 'gnu' OS.  Specifiy MIG
	when building for 'gnu' OS.