GN: Use build directory for CD for scripts

tools/gn/secondary/build/toolchain/mac/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-cc = "$relative_build_to_source_root_dir/third_party/llvm-build/Release+Asserts/bin/clang"
-cxx = "$relative_build_to_source_root_dir/third_party/llvm-build/Release+Asserts/bin/clang++"
+cc = to_build_path("//third_party/llvm-build/Release+Asserts/bin/clang")
+cxx = to_build_path("//third_party/llvm-build/Release+Asserts/bin/clang++")
 ld = cxx
 
 toolchain("clang") {
   tool("cc") {
     command = "$cc -MMD -MF \$out.d \$defines \$includes \$cflags \$cflags_c \$cflags_pch_c -c \$in -o \$out"
     description = "CC \$out"
     depfile = "\$out.d"
     deps = "gcc"
   }
   tool("cxx") {
@@ skipping 43 matching lines @@
   #}
   tool("stamp") {
     command = "\${postbuilds}touch \$out"
     description = "STAMP \$out"
   }
   tool("copy") {
     command = "ln -f \$in \$out 2>/dev/null || (rm -rf \$out && cp -af \$in \$out)"
     description = "COPY \$in \$out"
   }
 }