std.sync.atomic: extended atomic helper functions (#8866)

- deprecates `std.Thread.spinLoopHint` and moves it to `std.atomic.spinLoopHint`
- added an Atomic(T) generic wrapper type which replaces atomic.Bool and atomic.Int
- in Atomic(T), selectively expose member functions depending on T and include bitwise atomic methods when T is an Integer
- added fence() and compilerFence() to std.atomic

lib/std/Thread/Mutex.zig

@@ -126,7 +126,7 @@ pub const AtomicMutex = struct {
 
             var iter = std.math.min(32, spin + 1);
             while (iter > 0) : (iter -= 1)
-                std.Thread.spinLoopHint();
+                std.atomic.spinLoopHint();
         }
 
         new_state = .waiting;
@@ -149,7 +149,7 @@ pub const AtomicMutex = struct {
                         else => unreachable,
                     }
                 },
-                else => std.Thread.spinLoopHint(),
+                else => std.atomic.spinLoopHint(),
             }
         }
     }