all: basic support for the os package

The resulting binary is pretty big due to lacking optimizations
(probably because of interfaces), so that should be fixed.

src/runtime/sync.go

@@ -1 +1,13 @@
 package runtime
+
+// This file contains stub implementations for internal/poll.
+
+//go:linkname semacquire internal/poll.runtime_Semacquire
+func semacquire(sema *uint32) {
+	panic("todo: semacquire")
+}
+
+//go:linkname semrelease internal/poll.runtime_Semrelease
+func semrelease(sema *uint32) {
+	panic("todo: semrelease")
+}