Implement assume_aligned using the standard API

Rasmus Munk Larsenrequested to merge
rmlarsen1/eigen-new:assume_aligned into master

This implements Eigen::internal::assume_aligned to match the API for C++20 standard as best as possible using either std::assume_aligned or __builtin_assume_aligned if available. If neither is available, the function is a no-op.

The override macro EIGEN_ASSUME_ALIGNED was changed to EIGEN_DONT_ASSUME_ALIGNED, which now forces the function to be a no-op.

Edited by Rasmus Munk Larsen

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