Add prefetch to core

We are currently implementing a new algorithm in Boost.Histogram which is supposed to profit from cacheline prefetching. The prefetch instruction is a builtin, so we need code to get a platform-independent Boost command. I discovered that Boost.Context already has such an implementation in its details.

https://www.boost.org/doc/libs/1_81_0/boost/context/detail/prefetch.hpp

I propose to move this code to Core, so that all libraries can profit from prefetching. Currently, context and fiber use this code, and it is currently added to Boost.Histogram.