rspamd: fix builds on non-x86_64-linux platforms
rspamd: fix builds on non-x86_64-linux platforms

LuaJIT is built in rspamd only on x86_64-linux, and LuaJIT support
became enabled by default in 2.6, breaking builds without it. This
commit explicitly disables LuaJIT support on non-x86_64 architectures.

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