[libc] Improve get_object_files_for_test to reduce CMake configure time for tests. (#75552)

Profiling cmake shows that a significant time configuring `libc` folder
is spent on running `get_object_files_for_test` in the `test` folder (13
sec in `libc/test` folder / 16 sec in `libc` folder). By caching all
needed objects for each target instead of resolving every time, the time
cmake spends on configuring `libc/test` folder is reduced to ~1s.