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

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.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D93671

Author: shiltian

openmp/docs/SupportAndFAQ.rst

@@ -52,22 +52,11 @@ All patches go through the regular `LLVM review process
 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: