x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS

Extend spectre_v2= boot option with Kernel IBRS.

  [jpoimboe: no STIBP with IBRS]

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>