Don't use __builtin_alloca_with_align with nvc++

Morris Hafnerrequested to merge
mmha_nv/eigen:master into master

What does this implement/fix?

This patch fixes a compilation error with nvc++ when EIGEN_ALIGNED_ALLOCA is used. nvc++ does not support __builtin_alloca_with_align so EIGEN_ALIGNED_ALLOCA needs to use the fallback eigen_aligned_alloca_helper.

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