[libc++][Docs] Update benchmark doc wrt monorepo

Seems this section is not updated since we have transited to llvm-project monorepo.
At the start, we build libcxx under monorepo configuration but later try to make the separate configuration for libcxx build
and running benchmark.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D110722