[OpenMP] Allow to specify what plugins to look for (#74538)

By default we now only look for the plugins we build, but the user can
overwrite that with `LIBOMPTARGET_PLUGINS_TO_LOAD="cuda,amdgpu,x86_64"`

GitOrigin-RevId: 2a1bcf1f388c3a2924c5429c3f55be054f479950
3 files changed