[libc] Lay out framework for fuzzing libc functions.

Summary:
Added fuzzing test for strcpy and some documentation related to fuzzing.
This will be the first step in integrating this with oss-fuzz.

Reviewers: sivachandra, abrachet

Reviewed By: sivachandra, abrachet

Subscribers: gchatelet, abrachet, mgorny, MaskRay, tschuett, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D74091