powerpc/64s: Add support for a store forwarding barrier at kernel entry/exit

powerpc/64s: Add support for a store forwarding barrier at kernel entry/exit

This patch prevents a possible store forwarding between privilege
domains, by inserting a store forwarding barrier in kernel entry and
exit paths.

Barriers must be inserted generally before the first load after moving
to a higher privilege, and after the last store before moving to a
lower privilege, HV and PR privilege transitions must be protected.

Barriers are added as patch sections, with all kernel/hypervisor entry
points patched, and the exit points to lower privilge levels patched
similarly to the RFI flush patching.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
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>