push CROSS_COMPILE out to $(ARCH)_config.mk

Each arch should handle setting a proper default CROSS_COMPILE value in
their own config.mk file rather than having to maintain a large ugly list
in the Makefile.  By using conditional assignment, we don't have to worry
about the variable already being set (env/cmdline/etc...).

The common config.mk file takes care of exporting CROSS_COMPILE already,
and while a few variables (toolchain ones) utilize CROSS_COMPILE before
including the arch config.mk, they do so with deferred assignment.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>