[S390] mark disabled_wait as noreturn function

disabled_wait() won't return, so add an __attribute__((noreturn)).
This will remove a false positive finding which our internal code
checker reports.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>