kbuild: keep data tables through dead code elimination

When CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is enabled we must ensure
that we still keep various programatically-accessed tables.

[npiggin: Fold Paul's patches into one, and add a few more tables.
 diff symbol tables of allyesconfig with/without -gc-sections shows up
 lost tables quite easily.]

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.com>