MAKEALL: add support for per architecture toolchains

Add support for per architecture CROSS_COMPILE toolchain definitions
via CROSS_COMPILE_ARCH where "ARCH" is any of the supported u-boot
architectures.  This allows building every supported u-boot board in a
single pass of MAKEALL.

Signed-off-by: Allen Martin <amartin@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stephen Warren <swarren@wwwdotorg.org>