[libc] Switch functions to using global headers

This switches all of the files in src/string, src/math, and
test/src/math from using relative paths (e.g. `#include “include/string.h”`)
to global paths (e.g. `#include <string.h>`) to make bringing up those
functions on other platforms, such as fuchsia, easier.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D91394