import simd_ intrinsics

[folkertdev]

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 remove simd_fpow and simd_fpowi #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?