[libomptarget] Add cmake variables to disable building the amdgpu or cuda plugins

[JonChesterfield]

Summary

Analogous to the controls on building device runtimes

[JonChesterfield]

I'd like this for use on machines that have nvptx and amdgpu hardware available. It may also be useful for people working on the cpu side of openmp on a machine with a GPU present that they don't currently want running.

[jdoerfert]

LG, thought we should have a different control to disable plugins at runtime.

openmp/libomptarget/plugins/amdgpu/CMakeLists.txt

 ##===----------------------------------------------------------------------===##
 #
 #                     The LLVM Compiler Infrastructure
 #
 # This file is dual licensed under the MIT and the University of Illinois Open
 # Source Licenses. See LICENSE.txt for details.
 #
 ##===----------------------------------------------------------------------===##
 #
 # Build a plugin for an AMDGPU machine if available.
 #
 ##===----------------------------------------------------------------------===##
 
 ################################################################################
+set(LIBOMPTARGET_BUILD_AMDGPU_PLUGIN TRUE CACHE BOOL
+  "Whether to build AMDGPU plugin")
+if (NOT LIBOMPTARGET_BUILD_AMDGPU_PLUGIN)
+  libomptarget_say("Not building AMDGPU offloading plugin: LIBOMPTARGET_BUILD_AMDGPU_PLUGIN is false")
+  return()
+endif()
 
 # as of rocm-3.7, hsa is installed with cmake packages and kmt is found via hsa
 find_package(hsa-runtime64 QUIET 1.2.0 HINTS ${CMAKE_INSTALL_PREFIX} PATHS /opt/rocm)
 
 if(NOT LIBOMPTARGET_DEP_LIBELF_FOUND)
   libomptarget_say("Not building AMDGPU plugin: LIBELF not found")
   return()
 endif()

openmp/libomptarget/plugins/cuda/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 a plugin for a CUDA machine if available.
 #
 ##===----------------------------------------------------------------------===##
+set(LIBOMPTARGET_BUILD_CUDA_PLUGIN TRUE CACHE BOOL
+  "Whether to build CUDA plugin")
+if (NOT LIBOMPTARGET_BUILD_CUDA_PLUGIN)
+  libomptarget_say("Not building CUDA offloading plugin: LIBOMPTARGET_BUILD_CUDA_PLUGIN is false")
+  return()
+endif()
+
 if (NOT(CMAKE_SYSTEM_PROCESSOR MATCHES "(x86_64)|(ppc64le)|(aarch64)$" AND CMAKE_SYSTEM_NAME MATCHES "Linux"))
   libomptarget_say("Not building CUDA offloading plugin: only support CUDA in Linux x86_64, ppc64le, or aarch64 hosts.")
   return()
 elseif (NOT LIBOMPTARGET_DEP_LIBELF_FOUND)
   libomptarget_say("Not building CUDA offloading plugin: libelf dependency not found.")
   return()
 endif()