[libc] fix benchmark build

[SchrodingerZhu]

libc benchmark isn't really buildable in neither LLVM nor runtime build. The cmake code needs to be updated.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Schrodinger ZHU Yifan (SchrodingerZhu)

libc benchmark isn't really buildable in neither LLVM nor runtime build. The cmake code needs to be updated.