Move core:runtime to base:runtime; keep alias around

base/runtime/os_specific_freestanding.odin

@@ -0,0 +1,7 @@
+//+build freestanding
+package runtime
+
+// TODO(bill): reimplement `os.write`
+_os_write :: proc "contextless" (data: []byte) -> (int, _OS_Errno) {
+	return 0, -1
+}