[OpenMP] Make `libomptarget` link against `libomp`

In `libomptarget` we use a couple of functions from `libomp`, but we didn't link
`libomptarget` against `libomp`. That will not work on some platforms such
as macOS. A linker error will be encountered because those symbols are not resolved
at link time when building `libomptarget`. This patch simply makes `libomptarget`
link agains `libomp`, makes it a "user" of `libomp`. I think this will not break
the policies between `libomp` and `libomptarget`.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D149617