elan: wrap embedded linker

[Kha]

Motivation for this change

Lean 4 will soon start shipping its own toolchain including lld: leanprover/lean4#795. This change is in preparation of that by wrapping the embedded ld.lld (Linux)/ld64.lld (macOS) in order to pass the correct interpreter. While we're at it, we drop the old GMP wrapper that has not been necessary for a long time, if at all.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 21.11 Release Notes (or backporting 21.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[gebner]

Works for me. If anybody wants to test this (it would be great if someone could test this on darwin), this is what you should try:

echo 'int main() { return 42; }' >main.c
nix shell .#elan -c leanc +Kha/lean4:4.0.0-bundle-rc main.c
./a.out