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/development/libraries/wasilibc/default.nix

@@ -0,0 +1,28 @@
+{ stdenv, fetchFromGitHub, lib }:
+
+stdenv.mkDerivation {
+  name = "wasilibc-20190413";
+  src = fetchFromGitHub {
+    owner = "CraneStation";
+    repo = "wasi-sysroot";
+    rev = "079d7bda78bc0ad8f69c1594444b54786545ce57";
+    sha256 = "09s906bc9485wzkgibnpfh0mii7jkldzr1a6g8k7ch0si8rshi5r";
+  };
+  makeFlags = [
+    "WASM_CC=${stdenv.cc.targetPrefix}cc"
+    "WASM_NM=${stdenv.cc.targetPrefix}nm"
+    "WASM_AR=${stdenv.cc.targetPrefix}ar"
+    "INSTALL_DIR=${placeholder "out"}"
+  ];
+
+  postInstall = ''
+    mv $out/lib/*/* $out/lib
+  '';
+
+  meta = {
+    description = "WASI libc implementation for WebAssembly";
+    homepage    = "https://wasi.dev";
+    platforms   = lib.platforms.wasi;
+    maintainers = [ lib.maintainers.matthewbauer ];
+  };
+}