[RuntimeDyld] Make sure that RuntimeDyld regression tests only run for targets
that have been enabled.

Without this, testers will fail when llvm-rtdyld is invoked with triples for
unsupported targets.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211969 91177308-0d34-0410-b5e6-96231b3b80d8