darwin: add threading support and use it by default

src/internal/task/darwin.go

@@ -0,0 +1,11 @@
+//go:build darwin
+
+package task
+
+import "unsafe"
+
+// MacOS uses a pointer so unsafe.Pointer should be fine:
+//
+//	typedef struct _opaque_pthread_t *__darwin_pthread_t;
+//	typedef __darwin_pthread_t pthread_t;
+type threadID unsafe.Pointer