[libc++] Forward to std::{,w}memchr in std::find

Reviewed By: #libc, ldionne

Spies: Mordante, libcxx-commits, ldionne, mikhail.ramalho

Differential Revision: https://reviews.llvm.org/D144394

GitOrigin-RevId: 1fd08edd5808fdb642ecd2e612ae82d370a90163
17 files changed