Re-enable support for --program-prefix.

The Apple buildbots have been modified not to pass --target,
so they shouldn't choke on a default program prefix anymore.

Patch by Rick Foos!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164956 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed