remove dummy asm/kvm.h files

This patch removes the dummy asm/kvm.h files on architectures not (yet)
supporting KVM and uses the same conditional headers installation as
already used for a.out.h .

Also removed are superfluous install rules in the s390 and x86 Kbuild
files (they are already in Kbuild.asm).

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>