llvm and clang 6: factor out compiler-rt, fix libstdcxxStdenv sanitizer headers

Build compiler-rt separate from llvm and clang, and just add as an extra
library in the wrapper.

pkgs/development/compilers/llvm/6/clang/default.nix

@@ -30,10 +30,7 @@ let
       "-DSPHINX_OUTPUT_MAN=ON"
       "-DSPHINX_OUTPUT_HTML=OFF"
       "-DSPHINX_WARNINGS_AS_ERRORS=OFF"
-    ]
-    # Maybe with compiler-rt this won't be needed?
-    ++ stdenv.lib.optional stdenv.isLinux "-DGCC_INSTALL_PREFIX=${gcc}"
-    ++ stdenv.lib.optional (stdenv.cc.libc != null) "-DC_INCLUDE_DIRS=${stdenv.cc.libc}/include";
+    ];
 
     patches = [ ./purity.patch ];
 
@@ -51,13 +48,11 @@ let
     outputs = [ "out" "lib" "python" ];
 
     # Clang expects to find LLVMgold in its own prefix
-    # Clang expects to find sanitizer libraries in its own prefix
     postInstall = ''
       if [ -e ${llvm}/lib/LLVMgold.so ]; then
         ln -sv ${llvm}/lib/LLVMgold.so $out/lib
       fi
 
-      ln -sv ${llvm}/lib/clang/${release_version}/lib $out/lib/clang/${release_version}/
       ln -sv $out/bin/clang $out/bin/cpp
 
       # Move libclang to 'lib' output
@@ -79,7 +74,7 @@ let
     passthru = {
       isClang = true;
       inherit llvm;
-    } // stdenv.lib.optionalAttrs stdenv.isLinux {
+    } // stdenv.lib.optionalAttrs stdenv.targetPlatform.isLinux {
       inherit gcc;
     };