Summary:
We currently have an unnecessary level of indirection when initializing
the RPC client. This is a holdover from when the RPC client was not
trivially copyable and simply makes it more complicated. Here we use the
`asm` syntax to give the C++ variable a valid name so that we can just
copy to it directly.

Another advantage to this, is that if users want to piggy-back on the
same RPC interface they need only declare theirs as extern with the same
symbol name, or make it weak to optionally use it if LIBC isn't
avaialb.e

Summary:
Previous patches have made the `rpc.h` header independent of the `libc`
internals. This allows us to include it directly rather than providing
an indirect C API. This patch only does the work to move the header. A
future patch will pull out the `rpc_server` interface and simply replace
it with a single function that handles the opcodes.

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

Summary:
This variable needs a reserved name starting with `__`. It was
mistakenly changed with a mass replace. It happened to work because the
tests still picked up the associated symbol, but it just became a bad
name because it's not reserved anymore.

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

The RPC client must be initialized to set a pointer to the underlying
buffer. This is currently done with the `reset` method which may not be
ideal for the use-case. We want runtimes to be able to initialize this
without needing to call a kernel. Recent changes allowed the `Client`
type to be trivially copyable. That means we can create a client on the
server side and then copy it over. To that end we take the existing
externally visible symbol and initialize it to the client's pointer.
Therefore we can look up the symbol and copy it over once loaded.

No test currently, I tested with a demo OpenMP application but couldn't think of
how to put that in-tree.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D153633

Currently the implementation of the RPC interface requires a flexible
struct. This caused problems when compilling the RPC server with GCC as
would be required if trying to export the RPC server interface. This
required that we either move to the `x[1]` workaround or make it a
template parameter. While just using `x[1]` would be much less noisy,
this is technically undefined behavior. For this reason I elected to use
templates.

The downside to using templates is that the server code must now be able
to handle multiple different types at runtime. I was unable to find a
good solution that didn't rely on type erasure so I simply branch off of
the given value.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D153304

Summary:
These were accidentally added and don't do anything.