Clean up build-many-glibcs.py libcilkrts disabling.

Clean up build-many-glibcs.py libcilkrts disabling.

The disabling of libcilkrts in build-many-glibcs.py has some
peculiarities.  It's only for the final GCC build, not the initial
bootstrap one, whereas normally anything disabled for the final build
should be disabled for the bootstrap one as well.  And it's only for
Hurd, when it's more natural by analogy with the libsanitizer
disabling to disable this library unconditionally, not only for
targets where it's known to break.  This patch cleans up that
disabling accordingly, adding a comment so it's obvious it can be
removed once GCC 7 is too old to build glibc.

	* scripts/build-many-glibcs.py (Config.build_gcc): Use
	--disable-libcilkrts unconditionally, not just for the final GCC
	build for Hurd.