x86: Clean up IBRS functionality resident in common code

x86: Clean up IBRS functionality resident in common code

The IBRS and IBPB code and defines are resident in common code.
This patch moves them to x86 specific locations.

The patch also adds the command line options:
spectre_v2={on|off|auto}
nospectre_v2

on: unconditionally enable
off: unconditionally disable, no_spectre_v2, noibrs + noibpb
auto: default, on

Existing noibrs and noibpb will print a deprecated message

Orabug: 27353383
Signed-off-by: Kanth Ghatraju <kanth.ghatraju@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: John Haxby <john.haxby@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>