rcutorture: Add --kmake-arg argument to kvm.sh

This commit adds the --kmake-arg to kvm.sh, which allows passing in
things like "V=1" to see the build commands, as well as enabling the
CROSS_COMPILE= make macro used for cross-building.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Greg KH <gregkh@linuxfoundation.org>