[Bug c/92086] Provide way to avoid saving callee-saved registers in functions without callers

[fw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92086

--- Comment #3 from Florian Weimer <fw at gcc dot gnu.org> ---
It also saves stack space.

I'm not sure if it is prudent to repurpose noreturn+nothrow for this. There
might be existing such functions where people expect to see a full call stack. 
Something more explicit seems better.