<sys/cdefs.h>: Simplify the _Noreturn compatibility macro.

Our g++ doesn't support [[noreturn]] yet. We might use [[gnu::noreturn]]
here for now, but thinking about it, simpler is better, so just make it
__dead2 for anything that doesn't claim to be C11.

Buildworld-breakage-originally-pointed-out-by: sephe
Merge branch 'master' into gsocdrm_jfree