config.rocmSupport: init option

Parity with the CUDA option. Useful for example to get a coherent
python package set, without conflicting packages.

pkgs/top-level/config.nix

···
       feature = "build packages with CUDA support by default";
     };
 
+    rocmSupport = mkMassRebuild {
+      type = types.bool;
+      default = false;
+      feature = "build packages with ROCm support by default";
+    };
+
     showDerivationWarnings = mkOption {
       type = types.listOf (types.enum [ "maintainerless" ]);
       default = [];