CVE-2018-3639 (x86)

With the previous commit 50d36d375b89 ("x86/msr: Add definitions for
new speculation control MSRs") the spec control bits from upstream were
introduced, so rename the existing bits to match upstream:
  FEATURE_ENABLE_IBRS -> SPEC_CTRL_IBRS
  FEATURE_SET_IBPB -> PRED_CMD_IBPB
Signed-off-by: Juerg Haefliger <juergh@canonical.com>