Work around 32-bit Linux userlands.

- Move the gold binary to "gold64".
- Make "ld" a script that tests the userland bitness and executes the
  appropriate helper.

TEST=gold/ld --version is gold on my machine, but bfd linker when
I corrupt the string the script searches for.
Review URL: https://chromiumcodereview.appspot.com/9323038

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/gold@120368 4ff67af0-8c30-449e-8e8b-ad334ec8d88c