[Libomptarget] Output the DeviceRTL alongside the other libraries (#73705)

Summary:
Currently, the `libomp.so` and `libomptarget.so` are emitted in the
`./lib` build directory generally. This logic is internal to the
`add_llvm_library` function we use to build `libomptarget`. The
DeviceRTl static library however is in the middle of the OpenMP runtime
build, which can vary depending on if this is a runtimes or projects
build. This patch changes this to install the DeviceRTL static library
alongside the other OpenMP libraries so they are easier to find.