[OpenMP][NVPTX] Disable building NVPTX deviceRTL by default on a non-CUDA system

[tianshilei1992]

Summary

D95466 dropped CUDA to build NVPTX deviceRTL and enabled it by default.
However, the building requires some libraries that are not available on non-CUDA
system by default, which could break the compilation. This patch disabled the
build by default. It can be enabled with LIBOMPTARGET_BUILD_NVPTX_BCLIB=ON.

[kparzysz]

This compiles ok on my machine.

[jdoerfert]

also need to be backported to 12

openmp/libomptarget/deviceRTLs/nvptx/CMakeLists.txt

 ##===----------------------------------------------------------------------===##
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 ##===----------------------------------------------------------------------===##
 #
 # Build the NVPTX (CUDA) Device RTL if the CUDA tools are available
 #
 ##===----------------------------------------------------------------------===##
 
+# By default we will not build NVPTX deviceRTL on a non-CUDA
+set(LIBOMPTARGET_BUILD_NVPTX_BCLIB FALSE CACHE BOOL
+  "Whether build NVPTX deviceRTL on non-CUDA system.")
+
+if (NOT (LIBOMPTARGET_DEP_CUDA_FOUND OR LIBOMPTARGET_BUILD_NVPTX_BCLIB))
+  libomptarget_say("Not building NVPTX deviceRTL by default on non-CUDA system.")
+  return()
+endif()
+
 # Check if we can create an LLVM bitcode implementation of the runtime library
 # that could be inlined in the user application. For that we need to find
 # a Clang compiler capable of compiling our CUDA files to LLVM bitcode and
 # an LLVM linker.
 set(LIBOMPTARGET_NVPTX_CUDA_COMPILER "" CACHE STRING
   "Location of a CUDA compiler capable of emitting LLVM bitcode.")
 set(LIBOMPTARGET_NVPTX_BC_LINKER "" CACHE STRING
   "Location of a linker capable of linking LLVM bitcode objects.")