x86/spectre_v2: Figure out when to use IBRS.

x86/spectre_v2: Figure out when to use IBRS.

Which is if:
 a) on the bootline you have 'spectre_v2=ibrs' _and_ the microcode
    is loaded that exposes this. And nobody used 'noibrs'.

    Also if you have 'spectre_v2=ibrs noibrs' we end up falling
    in the 'lfence'. Unless somebody did 'spectre_v2=ibrs noibrs nolfence',
    then we go back to automatic discovery.

b) Kernel compiled without retpoline and the microcode is
   available.

c) Kernel compiled without retpoline and there is no microcode with IBRS
   then we pick 'lfence' on system calls.

Orabug: 27477743
CVE: CVE-2017-5715

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Pavel Tatashin <pasha.tatashin@oracle.com>