[OpenMP] Add documentation on using the `libc` in OpenMP

[jhuber6]

Summary

This points users to the libc documentation and explains the basics of
how it's used inside the runtime.

[jdoerfert]

We really need to split openmp/docs/design/Runtimes.rst.

Generally LG. Please add a sentence and a link to the supported functions page.
Also, remove the two I flagged below.

openmp/docs/design/Runtimes.rst

1355–1356

I would remove this sentence. It is too vague to be helpful.

1367–1368

Remove this, future looking statement w/o value now.

[jhuber6]

Addressing comments

openmp/docs/SupportAndFAQ.rst

 are not as well. Given that fact, the interfaces between 1) the Clang compiler
 and libomptarget, 2) the Clang compiler and device runtime library, and
 3) libomptarget and plugins are not guaranteed to be compatible with an earlier
 version. Users are responsible for ensuring compatibility when not using the
 Clang compiler and runtime libraries from the same build. Nevertheless, in order
 to better support third-party libraries and toolchains that depend on existing
 libomptarget entry points, contributors are discouraged from making
 modifications to them.
+
+Q: Can I use libc functions on the GPU?
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+LLVM provides basic ``libc`` functionality through the LLVM C Library. For 
+building instructions, refer to the associated `LLVM libc documentation 
+<https://libc.llvm.org/gpu/using.html#building-the-gpu-library>`_. Once built, 
+this provides a static library called ``libcgpu.a``. See the documentation for a 
+list of `supported functions <https://libc.llvm.org/gpu/support.html>`_ as well. 
+To utilize these functions, simply link this library as any other when building 
+with OpenMP.
+
+.. code-block:: shell
+
+   clang++ openmp.cpp -fopenmp --offload-arch=gfx90a -lcgpu
+
+For more information on how this is implemented in LLVM/OpenMP's offloading 
+runtime, refer to the `runtime documentation <libomptarget_libc>`_.

openmp/docs/design/Runtimes.rst

 LIBOMPTARGET_BLOCK_SIZE
 """""""""""""""""""""""
 This is the maximum size of a single message while streaming data transfers between the two endpoints and can be set for both endpoints.
 
 LIBOMPTARGET_RPC_LATENCY
 """"""""""""""""""""""""
 This is the maximum amount of time the client will wait for a response from the server.
 
+
+.. _libomptarget_libc:
+
+LLVM/OpenMP support for C library routines
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Support for calling standard C library routines on GPU targets is provided by 
+the `LLVM C Library <https://libc.llvm.org/gpu/>`_. This project provides two 
+static libraries, ``libcgpu.a`` and ``libllvmlibc_rpc_server.a``, which are used 
+by the OpenMP runtime to provide ``libc`` support. The ``libcgpu.a`` library 
+contains the GPU device code, while ``libllvmlibc_rpc_server.a`` provides the 
+interface to the RPC interface. More information on the RPC construction can be 
+found in the `associated documentation <https://libc.llvm.org/gpu/rpc.html>`_.
+
+To provide host services, we run an RPC server inside of the runtime. This 
+allows the host to respond to requests made from the GPU asynchronously. For 
+``libc`` calls that require an RPC server, such as printing, an external handle 
+to the RPC client running on the GPU will be present in the GPU executable. If 
+we find this symbol, we will initialize a client and server and run it in the 
+background while the kernel is executing.
+
+For example, consider the following simple OpenMP offloading code. Here we will 
+simply print a string to the user from the GPU.
+
+.. code-block:: c++
+
+   #include <stdio.h>
+
+   int main() {
+    #pragma omp target
+      { fputs("Hello World!\n", stderr); }
+   }
+
+We can compile this using the ``libcgpu.a`` library to resolve the symbols. 
+Because this function requires RPC support, this will also pull in an externally 
+visible symbol called ``__llvm_libc_rpc_client`` into the device image. When 
+loading the device image, the runtime will check for this symbol and initialize 
+an RPC interface if it is found. The following example shows the RPC server 
+being used.
+
+.. code-block:: console
+
+    $ clang++ hello.c -fopenmp --offload-arch=gfx90a -lcgpu
+    $ env LIBOMPTARGET_DEBUG=1 ./a.out
+    PluginInterface --> Running an RPC server on device 0
+    ...
+    Hello World!
+
 .. _libomptarget_device:
 
 LLVM/OpenMP Target Device Runtime (``libomptarget-ARCH-SUBARCH.bc``)
 --------------------------------------------------------------------
 
 The target device runtime is an LLVM bitcode library that implements OpenMP
 runtime functions on the target device. It is linked with the device code's LLVM
 IR during compilation.
 
 .. _libomptarget_dynamic_shared: