Add few docs and implementation of strcpy and strcat.
This patch illustrates some of the features like modularity we want
in the new libc. Few other ideas like different kinds of testing, redirectors
etc are not yet present.
Reviewers: dlj, hfinkel, theraven, jfb, alexshap, jdoerfert
Subscribers: mgorny, dexonsmith, llvm-commits
Differential Revision: https://reviews.llvm.org/D67867