Bump llama.cpp to b5686, fix build failures

[AsbjornOlling]

The main motivation for bumping llama.cpp is to close #747

In the meantime, there were some changes to the build-info.cpp generation. This caused a failure during the copy operation in build.rs.
Just removing the entire copy step seems to work. So I did that.

[AsbjornOlling]

Hold off on merging this, it seems like there could be an issue with windows builds

[AsbjornOlling]

I think I fixed it. At least the windows CI passes on my fork of this repo

[AsbjornOlling]

Hmph.

Actually now windows/vulkan builds are broken.

Vulkan builds work on linux and macos, though - and non-vulkan builds work on all platforms I have tested, including windows.

This PR is still strictly an improvment over the current main. On the current main branch, vulkan builds don't work for any platform. So I think it is safe to merge this, @MarcusDunn. I'll keep looking into fixing Windows/Vulkan.

[AsbjornOlling]

Actually now windows/vulkan builds are broken.

Seems like it may be caused by a path length limitation on windows :| Nevermind...

[AsbjornOlling]

I found a workaround for the windows/vulkan issue, but that introduces new problems. I had pushed my windows/vulkan workaround to this branch, but I rolled it back for now.

Let's just get llama.cpp bumped and the linux/vulkan stuff fixed. I'll make a separate PR for the other stuff 😇