Add prefetch_host

[TApplencourt]

This is PR who follows up on #277, and adds some prefetch_host API.

[ I don't think the addition of prefetch_host API was discussed by the committee, so sorry for opening the PR now. It's just that I got some free time and didn't want to forget about this ]

We have prefetch that allows the runtime to transfer data from the host to the device, but we are missing the reverse, transferring data from the device to the host.
Cuda and OpenCL have the API. As usual ( :) ) HipSYCL have already an extension for this: https://github.com/illuhad/hipSYCL/blob/develop/doc/extensions.md#hipsycl_ext_prefetch_host #9

[gmlueck]

Should this be a proposed PR for SYCL-Next instead? This repo is for fixes to SYCL 2020.

[TApplencourt]

I can do that. This adds a new API call so that it may be better in SYCL next indeed.
Should I make the PR to a new SYCLnext branch that I create?

[gmlueck]

Should I make the PR to a new SYCLnext branch that I create?

There is already a SYCL-Next branch on the internal GitLab repo. It should be an MR there.

[TApplencourt]

Closing it, will move to internal

[TApplencourt]

I think it's still a good idea (just got beaten by that today). Need to check if I did follow up and opened a issue on the internal

[TApplencourt]

Opened here https://gitlab.khronos.org/sycl/Specification/-/merge_requests/729 , need to be ported.

Question:

• Only free function?

[TApplencourt]

(Forgot about it, will port)

[TApplencourt]

@TApplencourt will do this afternoon