Summary:
This isn't used anymore, I moved the GPU extensions into `offload/`.

Summary:
RPC_ is a generic prefix here, use LIBC_ to indicate that these are
opcodes used to implement the C library

Summary:
I'm going to attempt to move the `rpc.h` header to a separate folder
that we can install and include outside of `libc`. Before doing this I'm
going to try to trim up the file so there's not as many things I need to
copy to make it work. This dependency on `cpp::functional` is a low
hanging fruit. I only did it so that I could overload the argument of
the work function so that passing the id was optional in the lambda,
that's not a *huge* deal and it makes it more explicit I suppose.

This reverts commit fea5914c926e2f013a8b5e27eaa74c7047fb2c71.

This reverts commit 069e8bcd82c4420239f95c7e6a09e1f756317cfc.

Summary:
Some tests failing, revert this for now.

Summary:
Currently there are several layers to handle `printf`. Since we now have
varargs and an implementation of `printf` this can be heavily
simplified.

1. The frontend renames `printf` into `omp_vprintf` and gives it an
   argument buffer.

Removing 1. triggered some code in the AMDGPU backend menat for HIP /
OpenCL, so I hadded an exception to it.

2. Forward this to CUDA vprintf or ignore it.

We no longer need special handling for it since we have varargs. So now
we just forward this to CUDA vprintf if we have libc, otherwise just
leave `printf` as an external function and expect that `libc` will be
linked in.

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

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