[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/X86.td b/llvm/lib/Target/X86/X86.td
index 7fc4def..d2055ad 100644
--- a/llvm/lib/Target/X86/X86.td
+++ b/llvm/lib/Target/X86/X86.td
@@ -134,6 +134,8 @@
 def FeatureVLX     : SubtargetFeature<"avx512vl", "HasVLX", "true",
                       "Enable AVX-512 Vector Length eXtensions",
                                       [FeatureAVX512]>;
+def FeaturePKU   : SubtargetFeature<"pku", "HasPKU", "true",
+                      "Enable protection keys">;
 def FeaturePCLMUL  : SubtargetFeature<"pclmul", "HasPCLMUL", "true",
                          "Enable packed carry-less multiplication instructions",
                                [FeatureSSE2]>;
@@ -491,6 +493,7 @@
   FeatureDQI,
   FeatureBWI,
   FeatureVLX,
+  FeaturePKU,
   FeatureCMPXCHG16B,
   FeatureSlowBTMem,
   FeaturePOPCNT,