[clang] [libc] [llvm] [openmp] [libc] Rework the GPU build to be a regular target (PR #81921)

[Vladimir Vereschaka via cfe-commits]

vvereschaka wrote:

@jhuber6 ,
looks like these changes break the following builds
* https://lab.llvm.org/buildbot/#/builders/235/builds/5630
* https://lab.llvm.org/buildbot/#/builders/232/builds/19808

there are a lot of CMake error messages started with 
```
CMake Error at cmake/modules/AddLLVM.cmake:631 (set_target_properties):
  set_target_properties called with incorrect number of arguments.
Call Stack (most recent call first):
  cmake/modules/AddLLVM.cmake:854 (llvm_add_library)
  lib/Transforms/Hello/CMakeLists.txt:13 (add_llvm_library)
-- Targeting X86
-- Targeting NVPTX
CMake Error at CMakeLists.txt:1213 (add_subdirectory):
  add_subdirectory given source "/unittest" which is not an existing
  directory.
CMake Error at tools/llvm-config/CMakeLists.txt:54 (string):
  string sub-command REGEX, mode MATCH needs at least 5 arguments total to
  command.
...
```
would you take care of it?

https://github.com/llvm/llvm-project/pull/81921