AIX: Drop unnecessary -l from CMAKE_DL_LIBS

The `-lld` linker flag was added from the previous command-line script
that have all values in `CMAKE_DL_LIBS` prepended with `-l`.

On AIX, the `dlopen()` is otherwise in C library (default). But it seems
that also `libld` library was once needed on some versions to have
dynamic loading working.

Sync `CMAKE_DL_LIBS` value style with all other current usages.