x86/bhi: Add BHI mitigation knob

Branch history clearing software sequences and hardware control
BHI_DIS_S were defined to mitigate Branch History Injection (BHI).

Add cmdline spectre_bhi={on|off|auto} to control BHI mitigation:

 auto - Deploy the hardware mitigation BHI_DIS_S, if available.
 on   - Deploy the hardware mitigation BHI_DIS_S, if available,
        otherwise deploy the software sequence at syscall entry and
	VMexit.
 off  - Turn off BHI mitigation.

The default is auto mode which does not deploy the software sequence
mitigation.  This is because of the hardening done in the syscall
dispatch path, which is the likely target of BHI.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>