x86/bugs/IBRS: Turn on IBRS in spectre_v2_select_mitigation

x86/bugs/IBRS: Turn on IBRS in spectre_v2_select_mitigation

instead of during early bootup. This makes the bootup much
faster as we may get an NMI (watchdog) during booting before we
make it to spectre_v2_select_mitigation - which means we would
be running with IBRS enabled.

OraBug: 28041771
CVE: CVE-2018-3639

Fixes: XYZ ("x86/bugs/IBRS: Use variable instead of defines for enabling IBRS")
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Mihai Carabas <mihai.carabas@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>