[OpenMP][Libomptarget][NFC] Use CMake Variables

This patchs adds CMake variables to add subdirectories and include
directories for libomptarget and explicitly gives the location of source
files.

Differential Revision: https://reviews.llvm.org/D93290

GitOrigin-RevId: 907886cc5b3a72250246f96bc6e6c11bd76b4cf6
2 files changed