Support AVX for i686.

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### Description
<!--Please explain your changes.-->

Support AVX for i686.

There was an existing work-around for windows.  Added the more generic
architecture comparison to also apply for linux.

### Reference issue
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You can link to a specific issue using the gitlab syntax #<issue number>.
If the MR fixes an issue, write "Fixes #<issue number>" to have the issue automatically closed on merge.
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Fixes #2991.

Closes #2991

See merge request libeigen/eigen!2037

(cherry picked from commit 8e60d4173c)

Eigen/src/Core/arch/AVX/PacketMath.h

@@ -2831,7 +2831,7 @@ inline __m128i segment_mask_4x8(Index begin, Index count) {
   mask <<= CHAR_BIT * count;
   mask--;
   mask <<= CHAR_BIT * begin;
-#if defined(_WIN32) && !defined(_WIN64)
+#if !EIGEN_ARCH_x86_64
   return _mm_loadl_epi64(reinterpret_cast<const __m128i*>(&mask));
 #else
   return _mm_cvtsi64_si128(mask);
@@ -2847,7 +2847,7 @@ inline __m128i segment_mask_8x8(Index begin, Index count) {
   mask <<= (CHAR_BIT / 2) * count;
   mask--;
   mask <<= CHAR_BIT * begin;
-#if defined(_WIN32) && !defined(_WIN64)
+#if !EIGEN_ARCH_x86_64
   return _mm_loadl_epi64(reinterpret_cast<const __m128i*>(&mask));
 #else
   return _mm_cvtsi64_si128(mask);