wasm: init cross target

Adds pkgsCross.wasm32 and pkgsCross.wasm64. Use it to build Nixpkgs
with a WebAssembly toolchain.

stdenv/cross: use static overlay on isWasm

isWasm doesn’t make sense dynamically linked.

pkgs/test/cross/default.nix

@@ -93,8 +93,7 @@ let
 
   };
 
-in (lib.mapAttrs (_: mapMultiPlatformTest builtins.id) tests)
-// (lib.mapAttrs' (name: test: {
-    name = "${name}-llvm";
-    value = mapMultiPlatformTest (system: system // {useLLVM = true;}) test;
-  }) tests)
+in {
+  gcc = (lib.mapAttrs (_: mapMultiPlatformTest (system: system // {useLLVM = false;})) tests);
+  llvm = (lib.mapAttrs (_: mapMultiPlatformTest (system: system // {useLLVM = true;})) tests);
+}