libargon2: fixed cross-compilation
libargon2: fixed cross-compilation

Makefile had a hardcoded unprefixed ar. I wrote a patch (sending it
upstream) and added an optional make flag to override it in case we're
cross-compiling.

Unfortunately, this causes a rebuild of native packages.

This commit also fixes the pkg-config file to be generated correctly,
patch was provided by @worldofpeace.

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