elf: fix DPTMOD64 relocations with null symbol

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>
pthread: add stubs for pthread_condattr_* functions

pthread_condattr_init() is needed for JDK8 to run. Add stub for now.

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>