libcxxabi: apply libcxxabi-wasm patch

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

@@ -13,7 +13,6 @@ stdenv.mkDerivation {
     "-DLLVM_ENABLE_LIBCXX=ON"
     "-DLIBCXXABI_USE_LLVM_UNWINDER=ON"
   ] ++ stdenv.lib.optionals stdenv.hostPlatform.isWasm [
-    "-DUNIX=ON"
     "-DLIBCXXABI_ENABLE_THREADS=OFF"
     "-DLIBCXXABI_ENABLE_EXCEPTIONS=OFF"
   ] ++ stdenv.lib.optionals (!enableShared) [
@@ -30,6 +29,8 @@ stdenv.mkDerivation {
     export TRIPLE=x86_64-apple-darwin
   '' + stdenv.lib.optionalString stdenv.hostPlatform.isMusl ''
     patch -p1 -d $(ls -d libcxx-*) -i ${../libcxx-0001-musl-hacks.patch}
+  '' + stdenv.lib.optionalString stdenv.hostPlatform.isWasm ''
+    patch -p1 -d $(ls -d llvm-*) -i ${./libcxxabi-wasm.patch}
   '';
 
   installPhase = if stdenv.isDarwin

pkgs/development/compilers/llvm/8/libcxxabi-wasm.patch

@@ -0,0 +1,16 @@
+diff --git a/cmake/modules/HandleLLVMOptions.cmake b/cmake/modules/HandleLLVMOptions.cmake
+index 15497d405e0..33f7f18193a 100644
+--- a/cmake/modules/HandleLLVMOptions.cmake
++++ b/cmake/modules/HandleLLVMOptions.cmake
+@@ -127,7 +127,10 @@ else(WIN32)
+       set(LLVM_HAVE_LINK_VERSION_SCRIPT 1)
+     endif()
+   else(FUCHSIA OR UNIX)
+-    MESSAGE(SEND_ERROR "Unable to determine platform")
++    if(${CMAKE_SYSTEM_NAME} MATCHES "Wasi")
++    else()
++      MESSAGE(SEND_ERROR "Unable to determine platform")
++    endif()
+   endif(FUCHSIA OR UNIX)
+ endif(WIN32)
+