Support AVX for i686.

<!-- 
Thanks for contributing a merge request!

We recommend that first-time contributors read our [contribution guidelines](https://eigen.tuxfamily.org/index.php?title=Contributing_to_Eigen).

Before submitting the MR, please complete the following checks:
- Create one PR per feature or bugfix,
- Run the test suite to verify your changes.
  See our [test guidelines](https://eigen.tuxfamily.org/index.php?title=Tests).
- Add tests to cover the bug addressed or any new feature.
- Document new features.  If it is a substantial change, add it to the [Changelog](https://gitlab.com/libeigen/eigen/-/blob/master/CHANGELOG.md).
- Leave the following box checked when submitting: `Allow commits from members who can merge to the target branch`.
  This allows us to rebase and merge your change.

Note that we are a team of volunteers; we appreciate your patience during the review process.
-->

### 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
<!--
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.
-->

Fixes #2991.

Closes #2991

See merge request libeigen/eigen!2037

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);