Fix implicit-fallthrough warnings in tst-setjmp.c

[Paul Eggert]

On 2/14/19 2:23 PM, Joseph Myers wrote:
>> Shouldn't we use the more-standard _Noreturn for this sort of thing? cdefs.h
>> defines _Noreturn for pre-C11 GCCs.
> The attribute seems much more widely used in glibc .c files at present.

Sure, but that habit predates C11. Now that we have a standard way to 
say "this function does not return" it's surely better to use the 
standard way, at least for new code and arguably even for old code.