Fix implicit-fallthrough warnings in tst-setjmp.c

[Paul Eggert]

On 2/14/19 2:03 PM, Joseph Myers wrote:
> -static void
> +static __attribute__ ((__noreturn__)) void

Shouldn't we use the more-standard _Noreturn for this sort of thing? 
cdefs.h defines _Noreturn for pre-C11 GCCs.