d/rules.real: Fix CROSS_COMPILE definition for hppa native build
debian/rules.real itself defines and uses CROSS_COMPILE to call the appropriate objcopy program for kernel code. When changing the way cross-tools are specified, I overlooked that this used the OVERRIDE_HOST_TYPE variable and would need to be updated. This broke native builds for hppa.
Change the logic for defining CROSS_COMPILE so that the KERNEL_{DEB_ARCH,GNU_TYPE} variables override DEB_HOST_{ARCH,GNU_TYPE} when set.