Revert "[libc][NFC][Obvious] Fix the benchmarks after the switch to llvm/third-party"

This reverts commit 39e9f5d3.

Reverting, as we needed to re-revert the benchmarks move because it was
causing a build failure in the Fuchsia bots due to the way they consume
libcxx's CMakeLists. I want to make sure I understand where the fix
should be for that. After that, I'll incorporate the change here in the
re-reland.