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.

lib/systems/examples.nix

@@ -116,7 +116,7 @@ rec {
     config = "aarch64-none-elf";
     libc = "newlib";
   };
-  
+
   aarch64be-embedded = {
     config = "aarch64_be-none-elf";
     libc = "newlib";
@@ -126,7 +126,7 @@ rec {
     config = "powerpc-none-eabi";
     libc = "newlib";
   };
-  
+
   ppcle-embedded = {
     config = "powerpcle-none-eabi";
     libc = "newlib";
@@ -211,4 +211,14 @@ rec {
     config = "x86_64-unknown-netbsd";
     libc = "nblibc";
   };
+
+  #
+  # WASM
+  #
+
+  wasi32 = {
+    config = "wasm32-unknown-wasi";
+    useLLVM = true;
+  };
+
 }