zip_build.py: use hardlinks on unix
zip_build.py: use hardlinks on unix

When zip_build archives the build, it creates two identical files:
an 'unversioned' copy and a 'versioned' copy.  Speed this up and reduce
space usage on unix by using a hard link instead of a copy.


Review URL: https://chromiumcodereview.appspot.com/11362102

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@166097 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build