Fix libcxxStdenv on Linux

This corrects linkage failures arising from missing -lc++abi and
resolves "unused argument" warnings arising due to -stdlib=libc++
serving no purpose when search paths are being supplied explicitly.

pkgs/development/compilers/llvm/3.5/libc++/setup-hook.sh

···
-export NIX_CFLAGS_COMPILE+=" -isystem @out@/include/c++/v1"
-
-export NIX_CXXSTDLIB_COMPILE=" -stdlib=libc++"
-export NIX_CXXSTDLIB_LINK=" -stdlib=libc++"
+export NIX_CXXSTDLIB_COMPILE+=" -isystem @out@/include/c++/v1"
+export NIX_CXXSTDLIB_LINK=" -lc++ -lc++abi"

pkgs/development/compilers/llvm/3.6/libc++/setup-hook.sh

···
-export NIX_CFLAGS_COMPILE+=" -isystem @out@/include/c++/v1"
-
-export NIX_CXXSTDLIB_COMPILE=" -stdlib=libc++"
-export NIX_CXXSTDLIB_LINK=" -stdlib=libc++"
+export NIX_CXXSTDLIB_COMPILE+=" -isystem @out@/include/c++/v1"
+export NIX_CXXSTDLIB_LINK=" -lc++ -lc++abi"

pkgs/top-level/all-packages.nix

···
     libc = glibc;
     binutils = binutils;
     inherit coreutils zlib;
-    extraPackages = [ libcxx ];
+    extraPackages = [ libcxx libcxxabi ];
     nativeTools = false;
     nativeLibc = false;
   };