std.Thread: Mutex and Condition improvements (#11497)

* Thread: minor cleanups

* Thread: rewrite Mutex

* Thread: introduce Futex.Deadline

* Thread: Condition rewrite + cleanup

* Mutex: optimize lock fast path

* Condition: more docs

* Thread: more mutex + condition docs

* Thread: remove broken Condition test

* Thread: zig fmt

* address review comments + fix Thread.DummyMutex in GPA

* Atomic: disable bitRmw x86 inline asm for stage2

* GPA: typo mutex_init

* Thread: remove noalias on stuff

* Thread: comment typos + clarifications

lib/std/Thread.zig

@@ -459,9 +459,8 @@ const UnsupportedImpl = struct {
     }
 
     fn unsupported(unusued: anytype) noreturn {
-        @compileLog("Unsupported operating system", target.os.tag);
         _ = unusued;
-        unreachable;
+        @compileError("Unsupported operating system " ++ @tagName(target.os.tag));
     }
 };
 
@@ -1188,27 +1187,3 @@ test "Thread.detach" {
     event.wait();
     try std.testing.expectEqual(value, 1);
 }
-
-fn testWaitForSignal(mutex: *Mutex, cond: *Condition) void {
-    mutex.lock();
-    defer mutex.unlock();
-    cond.signal();
-    cond.wait(mutex);
-}
-
-test "Condition.signal" {
-    if (builtin.single_threaded) return error.SkipZigTest;
-
-    var mutex = Mutex{};
-    var cond = Condition{};
-
-    var thread: Thread = undefined;
-    {
-        mutex.lock();
-        defer mutex.unlock();
-        thread = try Thread.spawn(.{}, testWaitForSignal, .{ &mutex, &cond });
-        cond.wait(&mutex);
-        cond.signal();
-    }
-    thread.join();
-}