Some objects have DPTMOD64 relocations with the null symbol, presumably to
set the value to 0 (it is too much trouble to write zero into the file during
the link phase, apparently).  Detect this condition and write the zero.

Needed by JDK8.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>