[unittest] Fix FrontendTests CanGenerateCC1CommandLineFlag when LLVM_DEFAULT_TARGET_TRIPLE is not normalized after D79796

`TargetOpts->Triple` is initialized as llvm::sys::getDefaultTargetTriple() which may not be normalized.
If LLVM_DEFAULT_TARGET_TRIPLE is powerpc64le-linux-gnu, we should check
check `-triple powerpc64le-linux-gnu`, instead of (normalized) `-triple powerpc64le-unknown-linux-gnu`