zorg: add redundancy to buildbot phases

Since xserve2 isn't coming up, add some redundancy to the phases that
use it.

<rdar://problem/15800410>

git-svn-id: https://llvm.org/svn/llvm-project/zorg/trunk@199036 91177308-0d34-0410-b5e6-96231b3b80d8