[Driver] BuildOffloadingActions: Actually stabilize iteration order
In
```
/tmp/StaticDebug/bin/clang -ccc-print-phases -lsomelib -fopenmp=libomp --target=powerpc64-ibm-linux-gnu -fopenmp-targets=x86_64-pc-linux-gnu,powerpc64-ibm-linux-gnu clang/test/Driver/openmp-offload.c clang/test/Driver/openmp-offload.c
```
Both ToolChains have one single empty arch. llvm::sort in
LLVM_ENABLE_EXPENSIVE_CHECKS=on builds could swap the two entries.
Fixes: 255986e27fcf9f0b36f7a23fbe030fcca1ba0249