Extend spectre_v2= boot option with Kernel IBRS.

  [jpoimboe: no STIBP with IBRS]
Signed-off-by: NPawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Reviewed-by: NJosh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: NBorislav Petkov <bp@suse.de>