Nix portability improvements

[hacker1024]

This MR updates the Nix package in the interest of improving portability (in regards to cross-compilation and other libcs).

Changes:

• Use a multi-output derivation to reduce runtime closure sizes
• Move dependencies into the native/propagated buildInput sets where appropriate
• Do not link stdc++fs on platforms that do not support it
• Allow setting LLAMA_BUILD_* options through Nix
• Enable tests in the Nix build
• Add myself as a Nix package maintainer

Tested platforms:

• GCC/glibc on x86_64 for x86_64
• LLVM/glibc on x86_64 for x86_64
• GCC/glibc on x86_64 for aarch64
• GCC/musl on x86_64 for static aarch64

[SomeoneSerge]

The .nix diff looks good; @hacker1024 would you be up to also run the tests? Speaking of, I still think it would make sense to add at least impure builds (i.e. build in a devShell) if not ccache to the CI, just that I'm occupied with other projects

[SomeoneSerge]

Should there be a special branch for https://releases.llvm.org/10.0.0/projects/libcxx/docs/UsingLibcxx.html#id2? Note that clang does not necessarily mean libc++, and gcc does not necessarily mean libstdc++ (e.g. in Nixpkgs all stdenvs use the same stdlib determined by the platform)

[SomeoneSerge]

Also idk about libdl handling

[hacker1024]

Potentially. I guess it depends on how many systems we actually want to support - the original change was required for GCC on RHEL 8, so there's a reason for this branch for that use case.

I'm not sure if it's worth preemptively handling ancient LLVM versions as well unless either someone explicitly requires it or we're actively testing it...

[SomeoneSerge]

GCC on RHEL 8,

Let's then add a comment explaining the "why"s

[philiptaron]

Tested. Nice. Let's do a nix flake update too. I have on my list to set up an automated PR system to push nix flake updates since #10526 landed.

[SomeoneSerge]

Tested. Nice. Let's do a nix flake update too. I have on my list to set up an automated PR system to push nix flake updates since #10526 landed.

We could also consider replacing the flake with normal Nix and e.g. following the release channel. EDIT: well we'd have to keep around a flake for compatibility, but we could e.g. leave out the lock file