[libomptarget-nvptx-bc] Pass found CUDA installations

[Hahnfeld]

Summary

We already know where the CUDA SDK is, so there is no point in
letting Clang search for it again and possibly finding no or
a different installation.

--cuda-path is supported since the beginning of CUDA support in
Clang, so making this required doesn't impose additional restrictions.

[gtbercea]

LGTM

[grokos]

LGTM

libomptarget/cmake/Modules/LibomptargetNVPTXBitcodeLibrary.cmake

if (${discard_cached} OR NOT DEFINED ${output})
     else()
       message(STATUS "Performing Test ${output} - Failed")
     endif()
   endif()
 endfunction()
 
 # These flags are required to emit LLVM Bitcode. We check them together because
 # if any of them are not supported, there is no point in finding out which are.
-set(compiler_flags_required -emit-llvm -O1 --cuda-device-only)
+set(compiler_flags_required -emit-llvm -O1 --cuda-device-only --cuda-path=${CUDA_TOOLKIT_ROOT_DIR})
 set(compiler_flags_required_src "extern \"C\" __device__ int thread() { return threadIdx.x; }")
 check_bitcode_compilation(LIBOMPTARGET_NVPTX_CUDA_COMPILER_SUPPORTS_FLAGS_REQUIRED "${compiler_flags_required_src}" ${compiler_flags_required})
 
 # It makes no sense to continue given that the compiler doesn't support
 # emitting basic LLVM Bitcode
 if (NOT LIBOMPTARGET_NVPTX_CUDA_COMPILER_SUPPORTS_FLAGS_REQUIRED)
   return()
 endif()