Describe the enhancement requested

Some of simd-related files are including concrete system headers such as:

However I see that https://github.com/apache/arrow/blob/6f64af54ea3662b7e1685b84a81429df14beace1/cpp/src/arrow/util/simd.h seems to be a centralized header to sort out all the platform-specific checks. Besides, I think it is a good place to put universal definitions - e.g. working around certain compiler bugs like what #44116 did - to be visible to all simd source files.

Therefore I think we can unify the simd header inclusion by utilizing simd.h.

Component(s)

C++