compiler: replace opencoded __attribute__((noreturn))

compiler: replace opencoded __attribute__((noreturn))

Make a formal define for noreturn in compiler.h, and fix up opencoded uses of
__attribute__((noreturn)).  This includes removing redundant uses with
function definitions which have a public declaration.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>