setarch: flush output streams before exec

setarch: flush output streams before exec

This fixes the following (non-deterministic) discrepancy:

$ setarch x86_64 -RFZLvXBIST3 --uname-2.6 echo success
Switching on READ_IMPLIES_EXEC.
Switching on ADDR_LIMIT_32BIT.
Switching on SHORT_INODE.
Switching on WHOLE_SECONDS.
Switching on STICKY_TIMEOUTS.
Switching on ADDR_LIMIT_3GB.
Switching on UNAME26.
success

$ setarch x86_64 -RFZLvXBIST3 --uname-2.6 echo success  &> /tmp/bla; cat /tmp/bla
success

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>