[HIP] Fix libclang_rt.builtins.a not found (#532)

* The solution to the libclang_rt builtins problem has changed with newer version of ROCm, as used by the Codeplay AMD plugin. The old suggested solution does not work.
* This PR:
  * Suggests a new solution, with accompanying minor CMake work.
  * Updates documentation.

cmake/FindCompiler.cmake

@@ -69,4 +69,15 @@ if(is_dpcpp)
       INTERFACE_LINK_LIBRARIES ${SYCL_LIBRARY})
   endif()
 
-endif()
+  if(ENABLE_ROCBLAS_BACKEND OR ENABLE_ROCRAND_BACKEND OR ENABLE_ROCSOLVER_BACKEND)
+    # Allow find_package(HIP) to find the correct path to libclang_rt.builtins.a
+    # HIP's CMake uses the command `${HIP_CXX_COMPILER} -print-libgcc-file-name --rtlib=compiler-rt` to find this path.
+    # This can print a non-existing file if the compiler used is icpx.
+    if(NOT HIP_CXX_COMPILER)
+      find_path(HIP_CXX_COMPILER clang++
+        HINTS ENV HIPROOT ENV ROCM_PATH
+      )
+    endif()
+  endif()
+
+endif(is_dpcpp)

docs/building_the_project_with_dpcpp.rst

@@ -457,11 +457,14 @@ Build FAQ
 
 clangrt builtins lib not found
   Encountered when trying to build oneMKL with some ROCm libraries. There are
-  several possible solutions: * If building Open DPC++ from source, add
-  ``compiler-rt`` to the external projects compile option:
-  ``--llvm-external-projects compiler-rt``. * The *clangrt* from ROCm can be
-  used, depending on ROCm version: ``export
-  LIBRARY_PATH=/path/to/rocm-$rocm-version$/llvm/lib/clang/$clang-version$/lib/linux/:$LIBRARY_PATH``
+  several possible solutions:
+
+  * If building Open DPC++ from source, add ``compiler-rt`` to the external
+    projects compile option: ``--llvm-external-projects compiler-rt``.
+  * Manually set the variable ``HIP_CXX_COMPILER`` to HIP's toolkit ``clang++``
+    path, for instance ``-DHIP_CXX_COMPILER=/opt/rocm/6.1.0/llvm/bin/clang++``.
+    oneMKL may fail to link if the clang versions of ``icpx`` and ``rocm`` are
+    not compatible.
 
 Could NOT find CBLAS (missing: CBLAS file)
   Encountered when tests are enabled along with the BLAS domain. The tests