Support cross build

Support cross build

The build system was not designed for cross compiling, so this
never worked. One problem is that we run both lopsubgen-stage1 and
lopsubgen during the build, so these executables must be built for
both the build and the host architecture.

This patch enables cross building by setting CC and CC_FOR_BUILD
like this:

	make CC=/usr/bin/arm-linux-gnueabihf-gcc-11 CC_FOR_BUILD=cc

Object files and executables for the build system go to build
directory while files for the host system stay in the top level
directory as before.