Add clang multilib variants (x64_64-only, 64/32bit), basic multilib tests

pkgs/development/compilers/llvm/multi.nix

@@ -0,0 +1,44 @@
+{ runCommand,
+clang,
+gcc64,
+gcc32,
+glibc_multi
+}:
+
+let
+  combine = basegcc: runCommand "combine-gcc-libc" {} ''
+    mkdir -p $out
+    cp -r ${basegcc.cc}/lib $out/lib
+
+    chmod u+rw -R $out/lib
+    cp -r ${basegcc.libc}/lib/* $(ls -d $out/lib/gcc/*/*)
+  '';
+  gcc_multi_sysroot = runCommand "gcc-multi-sysroot" {} ''
+    mkdir -p $out/lib/gcc
+
+    ln -s ${combine gcc64}/lib/gcc/* $out/lib/gcc/
+    ln -s ${combine gcc32}/lib/gcc/* $out/lib/gcc/
+    # XXX: This shouldn't be needed, clang just doesn't look for "i686-unknown"
+    ln -s $out/lib/gcc/i686-unknown-linux-gnu $out/lib/gcc/i686-pc-linux-gnu
+
+
+    # includes
+    ln -s ${glibc_multi.dev}/include $out/
+
+    # dynamic linkers
+    mkdir -p $out/lib/32
+    ln -s ${glibc_multi.out}/lib/ld-linux* $out/lib
+    ln -s ${glibc_multi.out}/lib/32/ld-linux* $out/lib/32/
+  '';
+
+  clangMulti = clang.override {
+    # Only used for providing expected structure re:dynamic linkers, AFAIK
+    # Most of the magic is done by setting the --gcc-toolchain option below
+    libc = gcc_multi_sysroot;
+
+    extraBuildCommands = ''
+      sed -e '$a --gcc-toolchain=${gcc_multi_sysroot}' -i $out/nix-support/libc-cflags
+    '';
+  };
+
+in clangMulti