Meta: Use the host CXX tool for linking as well

For example, if you set host_cxx=clang in your args.gn, we would still
use GCC for linking. This matches the CMake build now.

Meta/gn/build/toolchain/BUILD.gn

@@ -190,6 +190,7 @@ unix_toolchain("unix") {
     current_os = host_os
     cc = host_cc
     cxx = host_cxx
+    ld = host_cxx
   }
 }