[clang] [Driver] Silence stdlib warning when linking C on FreeBSD (PR #68011)

[via cfe-commits]

dankm wrote:

This fixes this inconsistency:

Linux:
```
$ clang -o test -stdlib=libc++ test.c
$ ./test
Hello world
```

FreeBSD:
```
$ clang -o test -stdlib=libc++ test.c
clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
$ ./test
Hello world
```

https://github.com/llvm/llvm-project/pull/68011