Add support for the BSD platforms

[prep]

This PR adds the ability to compile and run Ollama on various BSD platforms, specifically DragonFly, FreeBSD, NetBSD and OpenBSD. A couple notes:

1. It includes support for Vulkan to offload work to the GPU.
2. It's only been tested on OpenBSD, but hopefully interested parties in this community can test the other platforms.
3. ollama run is broken on OpenBSD. The readline package uses syscall which OpenBSD has removed from libc and the kernel. Instead, the readline package should be updated to use golang.org/x/sys/unix, similar to how ergochat/readline does it. I didn't want to include that work in this PR.
4. The tweak in gen_common.sh is to remove a bash'ism so that the file can be sourced from ksh.

On OpenBSD I needed to add these packages:

pkg_add cmake go vulkan-headers

The other platforms will probably have similarly named packages to install.