AVX512: _mm512_rsqrt28_ps is available for AVX512ER only

(grafted from 40b4bf3d32
)

Eigen/Core

@@ -171,6 +171,9 @@
       #ifdef __AVX512DQ__
         #define EIGEN_VECTORIZE_AVX512DQ
       #endif
+      #ifdef __AVX512ER__
+        #define EIGEN_VECTORIZE_AVX512ER
+      #endif
     #endif
 
     // include files

Eigen/src/Core/arch/AVX512/MathFunctions.h

@@ -375,7 +375,7 @@ prsqrt<Packet8d>(const Packet8d& _x) {
   // Insert NaNs and Infs in all the right places.
   return _mm512_mask_blend_pd(le_zero_mask, x, infs_and_nans);
 }
-#else
+#elif defined(EIGEN_VECTORIZE_AVX512ER)
 template <>
 EIGEN_STRONG_INLINE Packet16f prsqrt<Packet16f>(const Packet16f& x) {
   return _mm512_rsqrt28_ps(x);