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/compiler-rt.nix

@@ -60,7 +60,11 @@ stdenv.mkDerivation rec {
     ln -s $out/lib/*/clang_rt.crtend-*.o $out/lib/crtend.o
     ln -s $out/lib/*/clang_rt.crtbegin_shared-*.o $out/lib/crtbeginS.o
     ln -s $out/lib/*/clang_rt.crtend_shared-*.o $out/lib/crtendS.o
+  '' + stdenv.lib.optionalString stdenv.hostPlatform.isWasm ''
+    ln -s $out/lib/*/* $out/lib
   '';
 
+  NIX_LDFLAGS = stdenv.lib.optionalString stdenv.hostPlatform.isWasm "--allow-undefined --no-entry";
+
   enableParallelBuilding = true;
 }

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;
     };

pkgs/development/compilers/llvm/8/libc++/default.nix

@@ -1,4 +1,5 @@
-{ lib, stdenv, fetch, cmake, python, libcxxabi, fixDarwinDylibNames, version }:
+{ lib, stdenv, fetch, cmake, python, libcxxabi, fixDarwinDylibNames, version
+, enableShared ? true }:
 
 stdenv.mkDerivation rec {
   name = "libc++-${version}";
@@ -31,7 +32,11 @@ stdenv.mkDerivation rec {
     "-DLIBCXX_LIBCPPABI_VERSION=2"
     "-DLIBCXX_CXX_ABI=libcxxabi"
   ] ++ stdenv.lib.optional stdenv.hostPlatform.isMusl "-DLIBCXX_HAS_MUSL_LIBC=1"
-    ++ stdenv.lib.optional (stdenv.hostPlatform.useLLVM or false) "-DLIBCXX_USE_COMPILER_RT=ON";
+    ++ stdenv.lib.optional (stdenv.hostPlatform.useLLVM or false) "-DLIBCXX_USE_COMPILER_RT=ON"
+    ++ stdenv.lib.optional stdenv.hostPlatform.isWasm [
+      "-DLIBCXX_ENABLE_THREADS=OFF"
+      "-DLIBCXX_ENABLE_FILESYSTEM=OFF"
+    ] ++ stdenv.lib.optional (!enableShared) "-DLIBCXX_ENABLE_SHARED=OFF";
 
   enableParallelBuilding = true;
 
@@ -46,6 +51,6 @@ stdenv.mkDerivation rec {
     homepage = http://libcxx.llvm.org/;
     description = "A new implementation of the C++ standard library, targeting C++11";
     license = with stdenv.lib.licenses; [ ncsa mit ];
-    platforms = stdenv.lib.platforms.unix;
+    platforms = stdenv.lib.platforms.all;
   };
 }

pkgs/development/compilers/llvm/8/libc++abi.nix

@@ -1,4 +1,5 @@
-{ stdenv, cmake, fetch, libcxx, libunwind, llvm, version }:
+{ stdenv, cmake, fetch, libcxx, libunwind, llvm, version
+, enableShared ? true }:
 
 stdenv.mkDerivation {
   name = "libc++abi-${version}";
@@ -6,13 +7,20 @@ stdenv.mkDerivation {
   src = fetch "libcxxabi" "1k875f977ybdkpdnr9105wa6hccy9qvpd9xd42n75h7p56bdxmn2";
 
   nativeBuildInputs = [ cmake ];
-  buildInputs = stdenv.lib.optional (!stdenv.isDarwin && !stdenv.isFreeBSD) libunwind;
+  buildInputs = stdenv.lib.optional (!stdenv.isDarwin && !stdenv.isFreeBSD && !stdenv.hostPlatform.isWasm) libunwind;
 
   cmakeFlags = stdenv.lib.optionals (stdenv.hostPlatform.useLLVM or false) [
     "-DLLVM_ENABLE_LIBCXX=ON"
     "-DLIBCXXABI_USE_LLVM_UNWINDER=ON"
+  ] ++ stdenv.lib.optionals stdenv.hostPlatform.isWasm [
+    "-DUNIX=ON"
+    "-DLIBCXXABI_ENABLE_THREADS=OFF"
+  ] ++ stdenv.lib.optionals (!enableShared) [
+    "-DLIBCXXABI_ENABLE_SHARED=OFF"
   ];
 
+  patches = [ ./libcxxabi-no-threads.patch ];
+
   postUnpack = ''
     unpackFile ${libcxx.src}
     unpackFile ${llvm.src}
@@ -39,8 +47,9 @@ stdenv.mkDerivation {
     else ''
       install -d -m 755 $out/include $out/lib
       install -m 644 lib/libc++abi.a $out/lib
-      install -m 644 lib/libc++abi.so.1.0 $out/lib
       install -m 644 ../include/cxxabi.h $out/include
+    '' + stdenv.lib.optionalString enableShared ''
+      install -m 644 lib/libc++abi.so.1.0 $out/lib
       ln -s libc++abi.so.1.0 $out/lib/libc++abi.so
       ln -s libc++abi.so.1.0 $out/lib/libc++abi.so.1
     '';
@@ -50,6 +59,6 @@ stdenv.mkDerivation {
     description = "A new implementation of low level support for a standard C++ library";
     license = with stdenv.lib.licenses; [ ncsa mit ];
     maintainers = with stdenv.lib.maintainers; [ vlstill ];
-    platforms = stdenv.lib.platforms.unix;
+    platforms = stdenv.lib.platforms.all;
   };
 }

pkgs/development/compilers/llvm/8/libcxxabi-no-threads.patch

@@ -0,0 +1,12 @@
+diff --git a/CMakeLists.txt b/CMakeLists.txt
+index 4138acf..41b4763 100644
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -362,6 +362,7 @@ if (NOT LIBCXXABI_ENABLE_THREADS)
+                         " is also set to ON.")
+   endif()
+   add_definitions(-D_LIBCXXABI_HAS_NO_THREADS)
++  add_definitions(-D_LIBCPP_HAS_NO_THREADS)
+ endif()
+ 
+ if (LIBCXXABI_HAS_EXTERNAL_THREAD_API)