[X86] Enable EVEX512 when host CPU has AVX512 (#90479)

This is used when -march=native run on an unknown CPU to old version of
LLVM.

(cherry picked from commit b329179)

llvm/lib/TargetParser/Host.cpp

@@ -1266,8 +1266,10 @@ static void getAvailableFeatures(unsigned ECX, unsigned EDX, unsigned MaxLeaf,
     setFeature(X86::FEATURE_AVX2);
   if (HasLeaf7 && ((EBX >> 8) & 1))
     setFeature(X86::FEATURE_BMI2);
-  if (HasLeaf7 && ((EBX >> 16) & 1) && HasAVX512Save)
+  if (HasLeaf7 && ((EBX >> 16) & 1) && HasAVX512Save) {
     setFeature(X86::FEATURE_AVX512F);
+    setFeature(X86::FEATURE_EVEX512);
+  }
   if (HasLeaf7 && ((EBX >> 17) & 1) && HasAVX512Save)
     setFeature(X86::FEATURE_AVX512DQ);
   if (HasLeaf7 && ((EBX >> 19) & 1))
@@ -1772,6 +1774,7 @@ bool sys::getHostCPUFeatures(StringMap<bool> &Features) {
   Features["rtm"]        = HasLeaf7 && ((EBX >> 11) & 1);
   // AVX512 is only supported if the OS supports the context save for it.
   Features["avx512f"]    = HasLeaf7 && ((EBX >> 16) & 1) && HasAVX512Save;
+  Features["evex512"]    = Features["avx512f"];
   Features["avx512dq"]   = HasLeaf7 && ((EBX >> 17) & 1) && HasAVX512Save;
   Features["rdseed"]     = HasLeaf7 && ((EBX >> 18) & 1);
   Features["adx"]        = HasLeaf7 && ((EBX >> 19) & 1);