sandbox: don't set SA_NODEFER in signal handler

The sandbox can handle signals. Due to a damaged global data pointer
additional exceptions in the signal handler may occur leading to an endless
loop. In this case leave the handling of the secondary exception to the
operating system.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>