Fix standalone debug compilation on ARMv6.
Fix standalone debug compilation on ARMv6.

This commit moves a declaration of `ksKernelEntry` such that it is now only
guarded by `DEBUG`, not additionally `CONFIG_PRINTING`. This allows the kernel
to be built standalone with debugging enabled using the following invocation:

    DEBUG=1 TOOLPREFIX=arm-none-eabi- CPU=arm1136jf-s PLAT=imx31 ARCH=arm \
      ARMV=armv6 make

cmr/seL4,cmr/seL4,cmr/seL4