cpu: renamed cortexm_common->cortex-m3_common and cleanup

For better support of the different Cortex-M families I renamed the `coretexm_common` to `cortex-m3_common`. This makes the current implementation compatible with the proposed cortex-m0 implementation in #1291.

Though some parts of the Cortex-M implementations share the same code (e.g. `atomic_arch.c`, `irq_arch.c` and `cpu.h`), these files are small and simple and I think a duplication does not hurt. `thread_arch.c` holds the code that is different for each family, so I think it is the best solution to have separate folders for each of the cortex-m families.