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GH-44098: [C++] Add home made _mm256_set_m128i for compilers who are missing it (#44116)



### Rationale for this change

AVX2 intrinsic _mm256_set_m128i is missing in GCC versions <8.0 - this is a GCC bug discussed in [1], causing certain CI build failed.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80582

### What changes are included in this PR?

Check the GCC version and use a home made replacement if necessary.

### Are these changes tested?

Manually tested.

### Are there any user-facing changes?

None.

* GitHub Issue: #44098

Authored-by: Ruoxi Sun <zanmato1984@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>