gn build: Use rebase_path on filename args to libcxx/utils/gen_link_script.py

  $ ninja -j800
  [1/5] ACTION //libcxx/src:cxx_linker_script(//llvm/utils/gn/build/toolchain:stage2_unix)
  FAILED: lib/libc++.so
  python ../libcxx/utils/gen_link_script.py --input //build.gn/lib/libc++.so.0 --output //build.gn/lib/libc++.so c++abi unwind
  GENERATING SCRIPT: 'INPUT(libc++.so.0 -lc++abi -lunwind)' as file //build.gn/lib/libc++.so
  Traceback (most recent call last):
    File "../libcxx/utils/gen_link_script.py", line 57, in <module>
      sys.exit(main())
    File "../libcxx/utils/gen_link_script.py", line 50, in main
      with open(args.output, 'w') as f:
  IOError: [Errno 2] No such file or directory: '//build.gn/lib/libc++.so'
  ninja: build stopped: subcommand failed.

Differential revision: https://reviews.llvm.org/D65449

llvm-svn: 367314