Optimize atomics implementaiton for the Tile architecture


For atomic_counter and atomic_ptr classes, detect the Tile architecture
using #if defined __tile__ (matching the style of ARM and Solaris) and then
use the Tile atomic instructions. Without this change, the default Mutex
implementation is used, which is slower.

This only affects compiling on the 'tile' architecture (http://www.tilera.com).

Passes "make check"