[libc] Add basic fuzz target for the printf parser

The goal is to fuzz the entirety of printf, but the plan is to do it in
pieces for simplicity. This test fuzzes just the parser, while later
tests will fuzz the converters. This also adds a mock version of the
arg_list class.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D143784