[libomptarget] Make libomptarget.devicertl.a built in all cases. Make libomptarget.device.a built when using -DLLVM_ENABLE_PROJECTS=openmp Use add_custom_command. Reviewed By: jhuber6 Differential Revision: https://reviews.llvm.org/D128130 GitOrigin-RevId: 4d9499e8cc39080e50f75d6059737bcc5cc43847