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/compilers/llvm/8/default.nix

@@ -97,12 +97,14 @@ let
         targetLlvmLibraries.libcxx
         targetLlvmLibraries.libcxxabi
         targetLlvmLibraries.compiler-rt
+      ] ++ stdenv.lib.optionals (!stdenv.targetPlatform.isWasm) [
         targetLlvmLibraries.libunwind
       ];
       extraBuildCommands = ''
         echo "-target ${stdenv.targetPlatform.config}" >> $out/nix-support/cc-cflags
         echo "-rtlib=compiler-rt -Wno-unused-command-line-argument" >> $out/nix-support/cc-cflags
         echo "-B${targetLlvmLibraries.compiler-rt}/lib" >> $out/nix-support/cc-cflags
+      '' + stdenv.lib.optionalString (!stdenv.targetPlatform.isWasm) ''
         echo "--unwindlib=libunwind" >> $out/nix-support/cc-cflags
       '' + mkExtraBuildCommands cc;
     };