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.
You don't want the GC to mess with your lovely data structure?
Allocate on the stack, as a global static or let the GC know not to touch it.
Check Algol-68RS, Mesa/Cedar, Modula-2+, Modula-3, Oberon, Oberon-2, Active Oberon, Oberon-07.
Whether a particular language makes that nice enough to be more worth using than a non-GC language is another question.
This is only true of specific GC implementations. Incremental GCs never pause the program. On-the-fly and realtime GCs pause the program for a few microseconds.