Summary:
We start this thread if the RPC client symbol is detected in the loaded
binary. We should make this sleep if there's no work to avoid the thread
running at high priority when the (scarecely used) RPC call is actually
required. So, right now after 25 microseconds we will assume the server
is inactive and begin sleeping. This resets once we do find work.

AMD supports a more intelligent way to do this. HSA signals can wake a
sleeping thread from the kernel, and signals can be sent from the GPU
side. This would be nice to have and I'm planning on working with it in
the future to make this infrastructure more usable with existing AMD
workloads.

Co-authored-by: Joseph Huber <huberjn@outlook.com>

Summary:
This was originally added in as a hack to work around CUDA's limitation
on allocation. The `libc` implementation now isn't even used for CUDA so
this code is never hit. Even if this case, this code never truly worked.

A true solution would be to use CUDA's virtual memory API instead to
allocate 2MiB slabs independenctly from the normal memory management
done in the stream.

[Offload] Use new error code handling mechanism

This removes the old ErrorCode-less error method and requires
every user to provide a concrete error code. All calls have been
updated.

In addition, for consistency with error messages elsewhere in LLVM, all
messages have been made to start lower case.

Summary:
This patch moves the RPC server handling to be a header only utility
stored in the `shared/` directory. This is intended to be shared within
LLVM for the loaders and `offload/` handling.

Generally, this makes it easier to share code without weird
cross-project binaries being plucked out of the build system. It also
allows us to soon move the loader interface out of the `libc` project so
that we don't need to bootstrap those and can build them in LLVM.

Summary:
If the user deallocates an RPC device this can sometimes fail if the RPC
server is still running. This will happen if the modification happens
while the server is still checking it. This patch adds a mutex to guard
modifications to it.

Summary:
This patch just changes the interface to make starting the thread
multiple times permissable since it will only be done the first time.
Note that this does not refcount it or anything, so it's onto the user
to make sure that they don't shut down the thread before everyone is
done using it. That is the case today because the shutDown portion is
run by a single thread in the destructor phase.

Another question is if we should make this thread truly global state,
because currently it will be private to each plugin instance, so if you
have an AMD and NVIDIA image there will be two, similarly if you have
those inside of a shared library.

Summary:
Handling the RPC server requires running through list of jobs that the
device has requested to be done. Currently this is handled by the thread
that does the waiting for the kernel to finish. However, this is not
sound on NVIDIA architectures and only works for async launches in the
OpenMP model that uses helper threads.

However, we also don't want to have this thread doing work
unnnecessarily. For this reason we track the execution of kernels and
cause the thread to sleep via a condition variable (usually backed by
some kind of futex or other intelligent sleeping mechanism) so that the
thread will be idle while no kernels are running.

Somehow conflict with define in wingdi.h.

Fix build failures:

[ 52%] Building CXX object
projects/offload/plugins-nextgen/common/CMakeFiles/PluginCommon.dir/src/RPC.cpp.obj
In file included from
...llvm\offload\plugins-nextgen\common\src\RPC.cpp:16:
...\llvm\libc\shared\rpc.h(48,3): error: expected identifier
   48 |   ERROR = 0x1000,
      |   ^
c:\Program files (x86)\Windows
Kits\10\include\10.0.22000.0\um\wingdi.h(118,29): note: expanded from
macro 'ERROR'
  118 | #define ERROR               0
      |                             ^
...\llvm\offload\plugins-nextgen\common\src\RPC.cpp(75,17): error:
expected unqualified-id
   75 |     return rpc::ERROR;
      |                 ^
c:\Program files (x86)\Windows
Kits\10\include\10.0.22000.0\um\wingdi.h(118,29): note: expanded from
macro 'ERROR'
  118 | #define ERROR               0
      |                             ^
2 errors generated.