Blackfin arch: Remove all traces of the relocation stack

Remove all traces of the relocation stack.  It's been removed from
binutils for years now.

Add a sanity overflow check to pcrel24 relocations to catch modules that
were built without -mlong-calls.

Signed-off-by: Bernd Schmidt <bernds_cb1@t-online.de>
Signed-off-by: Bryan Wu <cooloney@kernel.org>