all: use a custom sync package

The sync package is strongly tied to the runtime, so it's easier to
implement a new one. Besides, it's pretty big so it's better to replace
it.

src/runtime/sync.go

@@ -1,13 +1 @@
 package runtime
-
-// This file contains support code for the sync package.
-
-//go:linkname registerPoolCleanup sync.runtime_registerPoolCleanup
-func registerPoolCleanup(cleanup func()) {
-	// Ignore.
-}
-
-//go:linkname notifyListCheck sync.runtime_notifyListCheck
-func notifyListCheck(size uintptr) {
-	// Ignore.
-}