[libc] add scanf entrypoints

This patch adds scanf, sscanf, and fscanf entrypoints. It also adds unit
tests for sscanf and a basic test to fscanf. The scanf function is
basically impossible to test in an automated fashion due to it recieving
user input.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D138076