[libomptarget] Add device RTL to regression test dependencies.

In a clean build directory, `check-openmp` or `check-libomptarget` will fail because of missing device RTL .bc files. Ensure that the new targets new custom targets `omptarget.devicertl.nvptx` and `omptarget.devicertl.amdgpu` (corresponding to the plugin rtl targets `omptarget.rtl.cuda`, respectively `omptarget.rlt.amdgpu` ) are dependencies of the regression tests.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D123177