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

[hans]

Summary

Otherwise it fails for me like this:

$ 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.

[thakis]

lgtm

llvm/utils/gn/secondary/libcxx/src/BUILD.gn

if (libcxx_enable_shared) {
   if (libcxx_enable_abi_linker_script) {
     action("cxx_linker_script") {
       script = "//libcxx/utils/gen_link_script.py"
       outputs = [
         "$runtimes_dir/libc++.so",
       ]
       args = [
         "--input",
-        "$runtimes_dir/libc++.so.0",
+        rebase_path("$runtimes_dir/libc++.so.0", root_build_dir),
         "--output",
-        "$runtimes_dir/libc++.so",
+        rebase_path("$runtimes_dir/libc++.so", root_build_dir),
         "c++abi",
         "unwind",
       ]
       deps = [
         ":cxx_symlink",
       ]
     }
   }