runtime: add dummy debug package

This package does not implement any methods, which is of course not
useful. However, by creating this package in advance it's possible to
see the next issue that's preventing something from building in TinyGo.

Motivated by: https://github.com/tinygo-org/tinygo/issues/1634

src/runtime/debug/debug.go

@@ -0,0 +1,2 @@
+// Package debug is a dummy package that is not yet implemented.
+package debug