kbuild: drop vmlinux dependency from "make install"

This removes the dependency from vmlinux to install, thus avoiding the
current situation where "make install" has a nasty tendency to leave
root-turds in the working directory.

It also updates x86-64 to be in sync with i386.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>