[Driver,test] Remove -v from mkdir & ln -s from a test since it fails on AIX (#72924)

The PR https://github.com/llvm/llvm-project/pull/70817 introduced a
small bug, the tests failed on ppc64-aix with:

```
RUN: at line 186: mkdir -pv /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/tools/clang/test/Driver/Output/darwin-header-search-libcxx.cpp.tmp/install/bin
+ mkdir -pv /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/tools/clang/test/Driver/Output/darwin-header-search-libcxx.cpp.tmp/install/bin
mkdir: illegal option -- v
Usage: mkdir [-p] [-e] [-m mode] Directory ... 
```

This PR removes the verbose flag from both `mkdir` and `ln -s`.