Don't expose unavailable cstdio functions.

Summary: These aren't available on Android in all configurations.

Reviewers: EricWF, mclow.lists, #libc, ldionne

Reviewed By: EricWF, #libc, ldionne

Subscribers: broadwaylamb, dexonsmith, ldionne, krytarowski, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D76093