[x86] adding PKU feature flag

the feature flag is essential for RDPKRU and WRPKRU instruction 
more about the instruction can be found in the SDM rev 56, vol 2 from http://www.intel.com/sdm

Differential Revision: http://reviews.llvm.org/D15491

llvm-svn: 255644
diff --git a/llvm/lib/Target/X86/X86Subtarget.cpp b/llvm/lib/Target/X86/X86Subtarget.cpp
index f90a0b0..8ef08c9 100644
--- a/llvm/lib/Target/X86/X86Subtarget.cpp
+++ b/llvm/lib/Target/X86/X86Subtarget.cpp
@@ -270,6 +270,7 @@
   HasBWI = false;
   HasVLX = false;
   HasADX = false;
+  HasPKU = false;
   HasSHA = false;
   HasPRFCHW = false;
   HasRDSEED = false;