[libc++] Disallow running the libc++ benchmarks in standalone builds

We are trying to remove duplication of third-party code in
https://reviews.llvm.org/D112012, which will move the Google
Benchmark code outside of the `libcxx/` directory. That breaks
running the benchmarks in the Standalone build. Since we have
deprecated the Standalone build anyway, this patch just removes
support for the benchmark in Standalone mode until we remove that
mode entirely.

Differential Revision: https://reviews.llvm.org/D113503