x86/cpufeature: Add feature bit for Intel's Silicon Debug CPUID bit

Add a CPUID feature bit for the SDBG (Silicon Debug) CPU feature
found on recent Intel systems starting with Haswell.

Using the IA32_DEBUG_INTERFACE MSR (index C80H) one can at least
detect if SDBG has been enabled by the firmware and if it has
been used or not.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Aaron Lu <aaron.lu@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dirk Brandewie <dirk.j.brandewie@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1437330403-12102-1-git-send-email-minipli@googlemail.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>