Rollup merge of #137551 - folkertdev:import-simd-intrinsics, r=RalfJung

import `simd_` intrinsics

In most cases, we can import the simd intrinsics rather than redeclare them. Apparently, most of these tests were written before `std::intrinsics::simd` existed.

There are a couple of exceptions where we can't yet import:

- the intrinsics are not declared as `const fn` in the standard library, causing issues in the `const-eval` tests
- the `simd_shuffle_generic` function is not exposed from `std::intrinsics`
- the `simd_fpow` and `simd_fpowi` functions are not exposed from `std::intrinsics` (removed in https://github.com/rust-lang/rust/pull/137595)
- some tests use `no_core`, and therefore cannot use `std::intrinsics`

r? ```@RalfJung```

cc ```@workingjubilee``` do you have context on why some intrinsics are not exposed?

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