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>
setarch: add shortcut binary uname26

There are some uname26 binaries flying around. Let's make it available
everywhere.

Internal SUSE references: fate313476, sr226509

CC: Stanislav Brabec <sbrabec@suse.cz>
Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>
Merge branch 'setarch' of https://github.com/rudimeier/util-linux

* 'setarch' of https://github.com/rudimeier/util-linux:
  setarch: add shortcut binary uname26
  setarch: flush output streams before exec