zstd: Fix build on riscv hosts

pkgs/tools/compression/zstd/default.nix

@@ -41,6 +41,8 @@ stdenv.mkDerivation rec {
       tests/playTests.sh
   '';
 
+  LDFLAGS = lib.optionalString stdenv.hostPlatform.isRiscV "-latomic";
+
   cmakeFlags = lib.attrsets.mapAttrsToList
     (name: value: "-DZSTD_${name}:BOOL=${if value then "ON" else "OFF"}") {
       BUILD_SHARED = !static;