Add an Architecture Abstraction Layer.

Currently, we support one architecture, but this provides a layer for
adding other architectures without adding more nested `#ifdef`s.

Fixes #42

src/ds/flaglock.h

@@ -13,7 +13,7 @@ namespace snmalloc
     FlagLock(std::atomic_flag& lock) : lock(lock)
     {
       while (lock.test_and_set(std::memory_order_acquire))
-        bits::pause();
+        AAL::pause();
     }
 
     ~FlagLock()