[Libomptarget] Consolidate CPU offloading into 'host' directory (#86014)

Summary:
All of these CPU targets use the same underlying implementation. We
should consolidate them into a single target to make it easier to update
this to a static library based approach. I have decided to call this the
'host' target so it can be given a single name. We still only build
these if the system processor matches and we are on Linux.