[libc] Add very basic stdio FILE and fwrite

Summary:
This patch adds a very basic `FILE` type and basic `fwrite`.

It also removes `snprintf` from `StdIO`'s function spec because `VarArgType` was causing the generation to fail.

Reviewers: sivachandra, PaulkaToast

Reviewed By: sivachandra

Subscribers: mgorny, MaskRay, tschuett, libc-commits

Differential Revision: https://reviews.llvm.org/D77626