headers: avoid bareword attributes

Always use the __-decorated form of an attribute name in public
headers, as the bareword form is in the user's namespace, and we
don't want compilation to break just because the user defines the
bareword to mean something else.

Signed-off-by: Eric Blake <eblake@redhat.com>

newlib/libc/sys/phoenix/sys/unistd.h

@@ -35,7 +35,7 @@
 
 extern char **environ;
 
-void _exit(int status) _ATTRIBUTE ((noreturn));
+void _exit(int status) _ATTRIBUTE ((__noreturn__));
 
 int	access(const char *pathname, int mode);
 unsigned int alarm(unsigned seconds);