On the Tile architecture, use atomic instructions for atomic ptr and counter.

For atomic_counter and atomic_ptr classes, detect the Tile architecture
using #if defined __tile__ matching ARM and Solaris and then use the
Tile atomic instructions. Without this change, the default Mutex
implementation is used, which is slower.
Merge pull request #535 from ken-tilera/master

Optimize atomics implementaiton for the Tile architecture