Upstream implementation reads the content of the SPEC_CTRL
MSR once during boot to record the state of reserved bits.
Any access to this MSR (to enable/disable IBRS) needs to
preserve those reserved bits.
This tries to catch and convert all occurrances of the
Intel based IBRS changes we carry.
CVE-2018-3639 (x86)
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>