rr: enable on aarch64-linux and disable on non-linux
rr: enable on aarch64-linux and disable on non-linux

According to https://github.com/rr-debugger/rr#system-requirements, rr only
works on Linux and works on aarch64 as of 5.6

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs