first entry, which is a special case) in rtld_start, because they could be
all 0s. Instead we use the difference between the real _DYNAMIC address
(which we can determine on 68k with a "lea (%pc,_DYNAMIC),..." and the
base-relative one (at the beginning of the GOT) to figure out the relocation
offset.
Not needed for binutils-current, but I might as well fix it now.