[libc] Add strchr implementation. Fixes bug in memchr.

Summary: [libc] Adds strchr implementation with unit tests. Fixes signed character bug in memchr.

Reviewers: sivachandra, PaulkaToast

Reviewed By: sivachandra

Subscribers: mgorny, tschuett, ecnelises, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D83353