[libc] Using llvm_libc memcpy in mem* benchmarks.

Currently the mem* benchmarks use memcpy from Glibc and memset from LLVM libc.
That's misleading and produces inconsistent results and behaviors.

This change makes Memcpy.cpp consistent with Memset.cpp:
https://github.com/llvm/llvm-project/blob/master/libc/benchmarks/Memset.cpp#L49

Reviewers: sivachandra

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D88271