https://en.wikipedia.org/wiki/Oberon_(operating_system)
http://wiki.osdev.org/Go_Bare_Bones
If I am able to write an OS using just the language, with the help of some Assembly, or bootstrap the language and runtime, it is a systems language.
Many of the criteria people use to judge systems languages like inline assembly, would disqualify C when applied to a pure ANSI C compliant compiler without language extensions.