[Offload] Enable multilib building for OpenMP/Offload (#188485)
Summary:
Right now the CMake does not follow the pattern other runtime projects
use. All this does is use the standard subdir to place libraries in a
unique location. This allows, for example, users to configure a debug
version of openmp / offload within the same CMake invocation.
---------
Co-authored-by: Michael Kruse <github@meinersbur.de>