Patch from Deepak Saxena

This patch adds some hooks to arch/arm/Makefile to enable proper building
of big-endian kernels.  It also adds an option to the the top level
arm KConfig that only allows a big-endian kernel to be built if the sub-arch
KConfig sets the CONFIG_ARCH_SUPPORTS_BIG_ENDIAN boolean to true. This is
mainly there to keep people from attemtpting to build a big-endian kernel
for a system that is not capable of running in big-endian mode.