internal/task: implement atomic primitives for preemptive scheduling

src/internal/task/pmutex-preemptive.go

@@ -0,0 +1,11 @@
+//go:build scheduler.threads
+
+package task
+
+// PMutex is a real mutex on systems that can be either preemptive or threaded,
+// and a dummy lock on other (purely cooperative) systems.
+//
+// It is mainly useful for short operations that need a lock when threading may
+// be involved, but which do not need a lock with a purely cooperative
+// scheduler.
+type PMutex = Mutex