stdenv: correct cross CMAKE_SYSTEM_NAME
stdenv: correct cross CMAKE_SYSTEM_NAME

As described in cmake cross instructions, CMAKE_SYSTEM_NAME should be
set to "Generic" if there is no applicable OS:

https://gitlab.kitware.com/cmake/community/wikis/doc/cmake/CrossCompiling#setting-up-the-system-and-toolchain

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