[libc] Move implementations of strcat and strcpy to the string directory.

Summary:
Now that tests live in separate top-level directory, keeping the
implementations of individual functions in a directory of their own is
not meaningful. Hence, this change moves them into the higher level
string directory.

NFC intended.

Reviewers: MaskRay

Subscribers: mgorny, tschuett, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D72295