Since commit 5d20ee31 ("kbuild: Allow LD_DEAD_CODE_DATA_ELIMINATION
to be selectable if enabled"), HAVE_LD_DEAD_CODE_DATA_ELIMINATION is
supposed to be selected by architectures that are capable of this
functionality.  LD_DEAD_CODE_DATA_ELIMINATION is now users' selection.
Update the help message.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>