rspamd: disable LuaJIT support on aarch64 [ZHF 19.93]

[WilliButz]

Motivation for this change

#68361

Things done

When compiled with LuaJIT support, rspamd segfaults on aarch64.
Without LuaJIT, rspamd falls back to plain Lua and torch support needs to
be disabled.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS (x86_64-linux & aarch64-linux)
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[avnik]

In theory luajit can be enabled for i686 as well, but I surprising, if someone use rspamd in production on i686

[vcunat]

Well, there are various workarounds to make LuaJIT work compatibly on aarch64, if you're really interested in that, but I believe there's apparently no standard/"clearly best" (in all cases) approach yet.