Revert "[libc++] Use CMake interface targets to setup benchmark flags"

This reverts commit da0592e4.

Reverting because this is incompatible with cmake 3.13.5, with the
minimum supported version being 3.13.4.

See https://luci-milo.appspot.com/p/fuchsia/builders/ci/clang-linux-x64/b8871967816877544224.