kvmtool: add script for updating kernel headers

From time to time (when new KVM kernel features get enabled in kvmtool),
we need to update the public kernel headers from a recent Linux tree.
Provide a script that makes sure we get the right files and that also
covers every architecture.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>