Update arm build to current sources
Need to define debug_handlers
so create arm-arch.h to declare it and
update arm-arch.c to define it. Finally, update arch.h to include
arm-arch.h.
Also, current sources use the xoroshiro RNG, so update the cross-compile script so that we build the xoroshiro RNG instead of the old mt-19937 RNG. This should work because we have a pure lisp implementation of the xoroshiro generator.