Prefetch instruction for Cranelift

#### Feature

Modern x86 and arm processors support prefetch instructions to ensure memory that's needed in the near future is in the cache.

#### Benefit

Improved performance for appropriately placed prefetches of needed memory.

#### Implementation

Since prefetches are nonsemantic, it should be relatively easy to implement. It may need some consideration in regards to spectre and the like though (which I don't know much about) and instruction reordering (to make sure the prefetch stays approximately the same "distance" to the actual load as in the ir).

#### Alternatives

Calling an external function that contains just the prefetch, which would incur overhead from the call.