[OpenMP][Docs] Updated the faq about building an OpenMP offloading capable compiler

[tianshilei1992]

Summary

After some issues about building runtimes along with LLVM were fixed,
building an OpenMP offloading capable compiler is pretty simple. This patch updates
the FAQ part in the doc.

[jdoerfert]

LGTM. This is really cool :)

[ye-luo]

It is lower case openmp not OPENMP.

CMake Error at runtimes/CMakeLists.txt:34 (message):
  LLVM_ENABLE_RUNTIMES requests OPENMP but directory not found:
  /home/yeluo/opt/llvm-clang/llvm-project/llvm/runtimes/../../OPENMP

[tianshilei1992]

In D93671#2468553, @ye-luo wrote:

It is lower case openmp not OPENMP.

CMake Error at runtimes/CMakeLists.txt:34 (message):
  LLVM_ENABLE_RUNTIMES requests OPENMP but directory not found:
  /home/yeluo/opt/llvm-clang/llvm-project/llvm/runtimes/../../OPENMP

I noticed the issue. Will submit a hot fix.

openmp/docs/SupportAndFAQ.rst

 <https://llvm.org/docs/Contributing.html#how-to-submit-a-patch>`_.
 
 
 .. _build_offload_capable_compiler:
 
 Q: How to build an OpenMP offload capable compiler?
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-To build an *effective* OpenMP offload capable compiler we recommend a two
-stage build. The first stage Clang does not require to be offload capable but
-all backends that are targeted by OpenMP need to be enabled. By default, Clang
-will be build with all backends enabled. This initial (stage 1) Clang is used
-to create a second Clang compiler that is offload capable as well as the
-:ref:`device runtime libraries <device_runtime>` that will be linked into the
-offloaded code to provide OpenMP runtime support on the device.
-
-Generic information about building LLVM is available `here
-<https://llvm.org/docs/GettingStarted.html>`__. The CMake options for the
-second stage Clang should include:
-
-- `LIBOMPTARGET_NVPTX_CUDA_COMPILER=$STAGE1/bin/clang` to use the stage one
-  compiler for the device runtime compilation.
-- `LIBOMPTARGET_NVPTX_ENABLE_BCLIB=ON` to enable efficient device runtimes in
-  bitcode format.
+To build an *effective* OpenMP offload capable compiler, only one extra CMake
+option, `LLVM_ENABLE_RUNTIMES="OPENMP"`, is needed when building LLVM (Generic
+information about building LLVM is available `here <https://llvm.org/docs/GettingStarted.html>`__.).
+Make sure all backends that are targeted by OpenMP to be enabled. By default,
+Clang will be build with all backends enabled.
 
 If your build machine is not the target machine or automatic detection of the
 available GPUs failed, you should also set:
 
 - `CLANG_OPENMP_NVPTX_DEFAULT_ARCH=sm_XX` where `XX` is the architecture of your GPU, e.g, 80.
 - `LIBOMPTARGET_NVPTX_COMPUTE_CAPABILITIES=YY` where `YY` is the numeric compute capacity of your GPU, e.g., 75.
 
 .. note::