[OpenMP] build offload plugins before testing them

Summary:
"make check-all" or "make check-libomptarget" would attempt to run offloading
tests before the offload plugins are built. This patch corrects that by adding
dependencies to the libomptarget CMake rules.

Reviewers: jdoerfert

Subscribers: mgorny, guansong, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D70803