Atomic - remove duplication and improve unsigned type support

The Atomic class is a platform-independent interface to platform-specific methods for instructions such as cas, xchg, cmpxchg, etc. The implementation contains unnecessary code duplication and the support for unsigned types is incomplete. As an example of the code duplication, the method
  cmpxchg_ptr(void *new_val, volatile void *dest, void *old_val)
has a body defined in each of the platform-specific files when a single platform-independent version would be sufficient.