x86: don't call wait4 as waitpid

Fix compilation on ARM:
pie/restorer.c: In function ‘wait_helpers’:
pie/restorer.c:728:3: error: implicit declaration of function ‘sys_waitpid’ [-Werror=implicit-function-declaration]
cc1: all warnings being treated as errors

Reported-by: Mr Jenkins
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>