Per the logic in top-level CMakeLists, `libomptarget` is placed into
`LLVM_LIBRARY_OUTPUT_INTDIR` when this variable is set. Adjust the test
logic to include this directory in `-L` and `-Wl,-rpath` arguments as
well, in order to fix finding tests when building via the `runtimes`
top-level directory.
Signed-off-by: Michał Górny <mgorny@gentoo.org>