[libc] Fix memcpy to adhere to qualified calls.

Summary: Switched to using the new memcpy implementation.

Reviewers: sivachandra, abrachet, gchatelet

Reviewed By: abrachet, gchatelet

Subscribers: mgorny, MaskRay, tschuett, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D77277