Support building "unix" port for ARM

This allows to build "unix" version for ARM hosts. Specifically tested on Ubuntu 12.04 armhf running in chroot on Samsung ARM Chromebook (Cortex-A15).

Having specific .cpu directive produces errors regarding A/M variants mismatch. There's nothing magic in nlrthumb.S, so it should not be required. But as I don't have cortex-m crosscompiler here, I just commented it out so far - please try STM port before merging.