[Libomptarget] Unify and simplify plugin CMake (#86191)

Summary:
This patch reworks the CMake handling for building plugins. All this
does is pull a lot of shared and common logic into a single helper
function.
This also simplifies the OMPT libraries from being built separately
instead of just added.
GitOrigin-RevId: dcbddc25250158469c5635ad2ae4095faef53dfd
6 files changed