The perror function writes an error message directly to stderr. This
patch adds an implementation, tests, and header generation details.

This is the first step in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450

Summary:
We have 'generic' implementations for some functions stdio functions.
The current logic mandates that all generic functions are implemented by
the target. This obviously isn't true and this caused the GPU builds to
fail once baremtal added some extra ones.

This patch changes the logic to always include the generic sources only
if they aren't already defined. This can probably be cleaned up and
formalized later, since this pattern is copied in many places, but for
now this fixes the failing GPU build bots.

This uses the templatized scanf Reader interface introduced in #131037.

Both fileno and fdopen require interfacing with the opaque FILE struct,
so they shouldn't be enabled in overlay mode. This patch moves both into
fullbuild only on all platforms.

Fixes #128643

Summary:
This patch implements the `printf` family of functions on the GPU using
the new variadic support. This patch adapts the old handling in the
`rpc_fprintf` placeholder, but adds an extra RPC call to get the size of
the buffer to copy. This prevents the GPU from needing to parse the
string. While it's theoretically possible for the pass to know the size
of the struct, it's prohibitively difficult to do while maintaining ABI
compatibility with NVIDIA's varargs.

Depends on https://github.com/llvm/llvm-project/pull/96015.

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken