[clang] [libc] [libc] Support opaque FILE* on baremetal (PR #168931)

[Petr Hosek via cfe-commits]

petrhosek wrote:

I'm thinking of combining the `fputc_internal`, `fputs_internal` and `fwrite_internal` into a single function/header (e.g. `write_internal`) to reduce the complexity. I'm planning on making the same changes to `fget*` and `*scanf*` family of functions in a follow up change.

https://github.com/llvm/llvm-project/pull/168931