Illustrate a redirector using the example of round function from math.h.

Setup demonstrated in this patch is only for ELF-ish platforms.

Also note:

1. Use of redirectors is a temporary scheme. They will be removed once
   LLVM-libc has implementations for the redirected functions.
2. Redirectors are optional. One can choose to not include them in the
   LLVM-libc build for their platform.
3. Even with redirectors used, we want to link to the system libc
   dynamically.

Reviewers: dlj, hfinkel, jakehehrlich, phosek, stanshebs, theraven, alexshap

Subscribers: mgorny, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D69020