stf-barrier: set eieio instruction bit 6 for future optimisations

stf-barrier: set eieio instruction bit 6 for future optimisations

Enable reserved instruction bit 6 in the eieio instruction; later a
faster version might be provided for this particular purpose (store
forward barrier) based on this hint.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Mauricio Faria de Oliveira <mauricfo@linux.vnet.ibm.com>
Signed-off-by: Breno Leitao <brenohl@br.ibm.com>
CVE-2018-3639 (powerpc)

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>